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IBPS RRB PO Efficiency Problems

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This page covers IBPS RRB PO Efficiency Problems with complete concept notes, 3 graded practice MCQs, key points and exam-specific tips. Free to study.

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Concept Notes

Efficiency Problems— Rules & Concept

Core ConceptRead this first — the foundation of the topic

Efficiency in Time & Work represents how much work a person can complete in unit time. Think of efficiency as work rate or speed of work. If Ram can finish a job in 10 days, his efficiency is 1/10 of the total work per day. This concept forms the backbone of most SSC CGL Time & Work problems. Key Rules: Total Work = Efficiency × Time. When workers combine, their efficiencies add up. If A's efficiency is 1/12 and B's efficiency is 1/15, together they work at (1/12 + 1/15) efficiency. The reciprocal of combined efficiency gives combined time.

Formula BlockMemorise — at least one formula appears in every paper
Individual Efficiency = 1/Time taken. Combined Efficiency = Sum of individual efficiencies. Time for combined work = 1/Combined Efficiency. Work Ratio = Efficiency Ratio = Inverse of Time Ratio.
Exam PatternsWhat examiners ask — read before attempting PYQs
Common types include

workers joining/leaving at different times, comparing work rates, finding individual efficiencies when combined time is given, and efficiency ratios. Questions often involve 2-3 workers with fractional efficiencies

Powerful Shortcut

LCM Method. Take LCM of all given times as total work units. Each person's efficiency becomes LCM/their time. This eliminates fractions completely and makes calculations super fast.

Worked ExampleSolve this step-by-step before moving on
1
Step 1

Find LCM of 15 and 20 = 60 (assume total work = 60 units)

2
Step 2

A's efficiency = 60/15 = 4 units per day

3
Step 3

B's efficiency = 60/20 = 3 units per day

4
Step 4

Combined efficiency = 4 + 3 = 7 units per day

5
Step 5

Combined time = 60/7 = 8(4/7) days Alternative method: A's efficiency = 1/15, B's efficiency = 1/20. Combined = 1/15 + 1/20 = (4+3)/60 = 7/60. Time = 1/(7/60) = 60/7 days. Another Trick: For two workers, combined time = (Product of individual times)/(Sum of individual times). Here: (15×20)/(15+20) = 300/35 = 60/7 days.

Exam TrapsCommon mistakes students make — avoid these

Students often forget that when efficiency increases, time decreases proportionally. Also, they add times instead of adding efficiencies when workers combine. Remember: efficiencies add up, not times.

Key Points to Remember

  • Efficiency = 1/Time taken by individual worker
  • When workers combine, their efficiencies get added
  • Combined time = 1/(Sum of individual efficiencies)
  • Work ratio equals efficiency ratio equals inverse of time ratio
  • LCM method eliminates fractions and speeds up calculations
  • For two workers: Combined time = (Product of times)/(Sum of times)
  • Higher efficiency means lower time taken for same work
  • Total work = Efficiency × Time for any worker

Exam-Specific Tips

  • Combined efficiency formula for two workers: 1/T1 + 1/T2
  • Work ratio formula: W1:W2 = E1×t1 : E2×t2
  • Efficiency is always expressed as fraction of total work per unit time
  • When efficiency doubles, time becomes half for same work
  • LCM method: Total work = LCM of all given individual times
  • Two workers formula: Combined time = (T1×T2)/(T1+T2)
  • If A is twice as efficient as B, then A:B efficiency ratio = 2:1
  • Efficiency and time are inversely proportional for same work
Practice MCQs

Efficiency Problems — Practice Questions

3graded MCQs · easy to hard · full solution & trap analysis

All MCQs →
Practice 1easy

The efficiency of worker X is twice that of worker Y. If X and Y together can finish a job in 9 days, how many days will X alone take to complete the job?

Practice 2easy

If 8 workers can build a wall in 15 days, how many days will 12 workers take to build the same wall (assuming all workers work at the same rate)?

Practice 3medium

X can complete 40% of a task in 8 days. Y can complete 60% of the same task in 12 days. If they work together, in how many days will the entire task be completed?

60-Second Revision — Efficiency Problems

  • Remember: Efficiencies add up when workers combine, times don't
  • Formula: Combined time for two workers = (T1×T2)/(T1+T2)
  • Trick: Use LCM method to avoid fractions in calculations
  • Trap: Don't confuse work ratio with time ratio - they're inverse
  • Quick check: Higher efficiency always means lower time
  • Pattern: Most SSC questions involve 2-3 workers with simple time values
  • Shortcut: Efficiency ratio = Inverse of time ratio
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