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NDA Boats & Streams

Study Material — 9 PYQs (2018–2020) · Concept Notes · Shortcuts

NDA Boats & Streams is a frequently tested subtopic — 9 previous year questions from 2018–2020 papers are included below with concept notes, key rules and shortcut tricks.

9 PYQs
2018–2020
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10 Key Points
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Previous Year Questions

NDA Boats & Streams — Past Exam Questions

9 questions from actual NDA papers · all shown free · click option to reveal solution

Exam Q 12018Previous Year Pattern

A boat travels 36 km downstream in 2 hours and the speed of the stream is 3 km/h. What is the speed of the boat in still water?

Exam Q 22020Previous Year Pattern

A boat travels 48 km downstream in 3 hours and 48 km upstream in 6 hours. What is the speed of the boat in still water?

Exam Q 32019Previous Year Pattern

A boat travels 48 km downstream in 3 hours and 48 km upstream in 6 hours. What is the speed of the boat in still water?

Exam Q 42020Previous Year Pattern

A boat travels 48 km downstream in 3 hours and 48 km upstream in 4 hours. A man swims at a constant speed in still water. If the man swims the same 48 km downstream, how long will it take him (in hours)?

Exam Q 52019Previous Year Pattern

A boat travels 48 km downstream in 3 hours and 48 km upstream in 6 hours. A man swims at a constant speed in still water. What is the speed of the current?

Exam Q 62018Previous Year Pattern

A boat travels 48 km downstream in 3 hours and the same distance upstream in 4 hours. If the boat is to travel from point A to point B (84 km apart) downstream and then return to point A, what is the total time taken for the round trip?

Exam Q 72018Previous Year Pattern

A boat takes 6 hours to travel 120 km downstream and 10 hours to travel the same distance upstream. If the boat must travel 240 km downstream and then immediately return 240 km upstream, what is the total time taken for the entire journey?

Exam Q 82019Previous Year Pattern

A boat travels 48 km downstream in 3 hours and 48 km upstream in 6 hours. A man swims at a constant speed in still water. If the man swims the same 48 km downstream, he takes 4 hours. What is the ratio of the man's swimming speed to the boat's speed in still water?

Exam Q 92020Previous Year Pattern

A boat travels 48 km downstream in 3 hours and 48 km upstream in 6 hours. A swimmer can swim at a speed equal to the boat's speed in still water. If the swimmer swims downstream for 2 hours and then upstream for 4 hours in the same river, what is the total distance covered by the swimmer?

Concept Notes

Boats & Streams— Rules & Concept

1.Core Concept

Read this first — the foundation of the topic

BOATS & STREAMS — COMPLETE GUIDE FOR SSC CGL ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

CORE CONCEPT — WHAT IS IT? ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Imagine a boat moving on a river. The river has its own current (flow of water). When the boat moves in the same direction as the river, the water helps it go faster. When the boat moves against the river, the water slows it down. This is the entire concept of Boats and Streams. It is just a real-life version of relative speed.

Three things you must know: • Speed of Boat in Still Water (B) — how fast the boat moves if there is no current at all.

• Speed of Stream/Current (S) — how fast the river water flows. • Upstream / Downstream — the two directions of travel.

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

2.Key Rules

Core rules you must know cold

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ • DOWNSTREAM means the boat moves WITH the current. Water helps. Speed increases. • UPSTREAM means the boat moves AGAINST the current. Water resists.

Speed decreases. • Still Water means no current at all. Stream speed = 0. ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

3.Formula Block

Memorise — at least one formula appears in every paper
Let B = Speed of Boat in Still Water, S = Speed of Stream
1. Downstream Speed (D) = B + S
2. Upstream Speed (U) = B - S
3. Speed of Boat in Still Water = (D + U) / 2
4. Speed of Stream = (D - U) / 2
5. Distance = Speed x Time (always applicable)
Memory Trick: D is Downstream = D for 'Double help' (boat + stream). U is Upstream = U for 'Uphill fight' (boat - stream).

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

4.Exam Patterns

What examiners ask — read before attempting PYQs

— WHAT GETS ASKED ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ SSC CGL asks 1 to 2 questions from this topic almost every year. Common question types: • Find upstream or downstream speed given boat and stream speed. • Find time to travel a given distance upstream or downstream. • Find speed of boat or stream when upstream/downstream speeds are given. • Find total time for a round trip (upstream + downstream). • Find distance when time difference between upstream and downstream is given. ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

5.Shortcuts

Use these to save 30–60 seconds per question
TRICK 2 — Round Trip Time

Total time for round trip over distance 'd' = d/(B+S) + d/(B-S) Simplified: Total time = 2 x d x B / (B^2 - S^2) Use this when question asks total time for going and coming back

TRICK 3 — Time Difference Trick

If a boat takes 't' hours more to go upstream than downstream over same distance 'd': d x 2S = t x (B^2 - S^2) Use this to find distance or speed when time difference is given. ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

6.Worked Example

Solve this step-by-step before moving on
1
Step 1

Find downstream speed. Downstream speed = Distance / Time = 12 / 2 = 6 km/h

2
Step 2

Find upstream speed. Upstream speed = 8 / 4 = 2 km/h

3
Step 3

Apply formula. Speed of Boat = (6 + 2) / 2 = 8 / 2 = 4 km/h Speed of Stream = (6 - 2) / 2 = 4 / 2 = 2 km/h Answer: Boat speed = 4 km/h, Stream speed = 2 km/h ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ WORKED EXAMPLE 2 — Medium Level (Round Trip) ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Question: A boat travels 30 km upstream and 30 km downstream. Speed of boat in still water is 10 km/h and stream speed is 2 km/h. Find total time taken.

1
Step 1

Find upstream speed. Upstream = 10 - 2 = 8 km/h

2
Step 2

Find downstream speed. Downstream = 10 + 2 = 12 km/h

3
Step 3

Find time for each leg. Time upstream = 30 / 8 = 3.75 hours Time downstream = 30 / 12 = 2.5 hours

4
Step 4

Total time = 3.75 + 2.5 = 6.25 hours = 6 hours 15 minutes Answer: 6 hours 15 minutes ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

7.Exam Traps

Common mistakes students make — avoid these

— NUMBER 1 TRAP IN EXAM ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Most students confuse UPSTREAM and DOWNSTREAM speed with BOAT speed and STREAM speed. They directly use the upstream speed as the stream speed or vice versa. Remember: Upstream and downstream are COMBINED speeds (boat plus or minus stream). You MUST apply the average formulas to separate them.

Never treat D or U directly as boat speed or stream speed. This single mistake causes wrong answers in at least 40% of students.

Key Points to Remember

  • Downstream Speed = Speed of Boat + Speed of Stream (water helps the boat).
  • Upstream Speed = Speed of Boat - Speed of Stream (water opposes the boat).
  • Formula: Speed of Boat in Still Water = (Downstream Speed + Upstream Speed) / 2.
  • Formula: Speed of Stream = (Downstream Speed - Upstream Speed) / 2.
  • If a boat travels same distance both ways, it always takes MORE time upstream than downstream.
  • Round Trip Formula: Total Time = 2 x Distance x Boat Speed / (Boat Speed^2 - Stream Speed^2).
  • If stream speed equals boat speed, upstream speed becomes ZERO — boat cannot move upstream.
  • Distance = Speed x Time applies to every sub-question in Boats and Streams without exception.
  • Shortcut: Add downstream and upstream speeds and halve to get boat speed instantly in exam.
  • Trap: Never confuse downstream/upstream speed with the speed of stream — they are NOT the same thing.

Exam-Specific Tips

  • Downstream Speed formula: D = B + S, where B is boat speed and S is stream speed.
  • Upstream Speed formula: U = B - S, where B is boat speed and S is stream speed.
  • Speed of Boat in Still Water = (D + U) / 2 — this is the most frequently tested derivation formula.
  • Speed of Stream = (D - U) / 2 — directly tested as a standalone MCQ in SSC CGL and CHSL.
  • If a boat takes time T1 downstream and T2 upstream for the same distance, then D/U = T2/T1 (inverse ratio of times equals ratio of speeds).
  • Round trip total time formula: 2dB / (B^2 - S^2), where d is one-way distance.
  • A boat that covers equal distances upstream and downstream does NOT have an average speed of B — actual average speed = (B^2 - S^2) / B, which is always less than B.
  • If the question states a man rows at 'x km/h in still water' and stream is 'y km/h', downstream = x+y and upstream = x-y — this direct substitution is tested repeatedly.
Practice MCQs

Boats & Streams — Practice Questions

41graded MCQs · easy to hard · full solution & trap analysis · showing 20 of 41

All MCQs →
Practice 1easy

A boat travels 60 km downstream in 4 hours and 60 km upstream in 6 hours. What is the speed of the stream?

Practice 2easy

A boat's speed in still water is 10 km/h and the stream speed is 3 km/h. How long will it take to travel 91 km upstream?

Practice 3easy

A boat travels 48 km downstream in 3 hours. If the speed of the stream is 2 km/h, what is the speed of the boat in still water?

Practice 4easy

A boat's speed in still water is 10 km/h and the stream's speed is 2 km/h. If the boat travels 36 km downstream and then returns to the starting point, what is the total time taken?

Practice 5easy

A boat takes 5 hours to travel 60 km upstream. If the boat's speed in still water is 14 km/h, what is the speed of the stream?

Practice 6easy

A man can row 30 km downstream in 5 hours. The stream speed is 1 km/h. What is his speed in still water?

Practice 7easy

A man can row a boat at 8 km/h in still water. The stream flows at 3 km/h. How much longer does it take him to row 55 km upstream compared to rowing 55 km downstream?

Practice 8easy

A boat's speed in still water is 9 km/h and the stream speed is 2 km/h. If the boat travels 77 km upstream, how much time does it take?

Practice 9easy

A boat's speed in still water is 10 km/h and the stream's speed is 3 km/h. How long will it take the boat to travel 91 km upstream?

Practice 10easy

A boat covers 80 km downstream in 4 hours and the same distance upstream in 5 hours. What is the speed of the stream?

Practice 11easy

A man can row 5 km/h in still water. The river flows at 1 km/h. If he rows upstream for 2 hours and then downstream for 2 hours, what is the total distance covered?

Practice 12easy

A boat travels 48 km downstream in 3 hours. If the speed of the stream is 2 km/h, what is the speed of the boat in still water?

Practice 13easy

A boat takes 4 hours to travel 60 km downstream and 6 hours to travel the same distance upstream. What is the speed of the stream?

Practice 14easy

A boat takes 5 hours to travel 60 km upstream. If the boat's speed in still water is 14 km/h, what is the speed of the stream?

Practice 15easy

The speed of a boat in still water is 9 km/h. It travels 45 km downstream in 3 hours. What is the speed of the stream?

Practice 16medium

The speed of a boat in still water is 15 km/h and the speed of the stream is 3 km/h. How much time will the boat take to cover 72 km downstream and then return to the starting point?

Practice 17medium

A boat can travel 35 km upstream in 5 hours. If the stream's speed is 2 km/h, what is the speed of the boat in still water?

Practice 18medium

A man can row 30 km downstream in 3 hours and the same distance upstream in 5 hours. What is the speed of the current?

Practice 19medium

A swimmer swims 24 km downstream in 2 hours and 24 km upstream in 4 hours. If the swimmer swims in still water for 6 hours at the same pace, how far will he travel?

Practice 20medium

A boat travels 60 km downstream in 4 hours. The speed of the current is 3 km/h. How much time will the boat take to travel 60 km upstream?

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60-Second Revision — Boats & Streams

  • Formula: Downstream = Boat + Stream. Upstream = Boat - Stream. Memorise this first.
  • Formula: Boat Speed = (D + U) / 2. Stream Speed = (D - U) / 2. Use these to find unknowns instantly.
  • Trap: Upstream and downstream values are NOT the individual speeds — always extract boat and stream using the average formulas.
  • Remember: Upstream takes more time, downstream takes less time for the same distance — use this to check your answer.
  • Round Trip: Use formula 2dB / (B^2 - S^2) when question asks total time for going and returning over same distance.
  • Time and Distance: Always break the question into small steps — find speed first, then apply Distance = Speed x Time.
  • Quick Check: If stream speed is greater than boat speed, upstream speed is negative — that means the boat cannot move against the current. Mark such options carefully.
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