A is three times more efficient as a worker than B, which allows A to complete a job in 60 days less than B. If they work together, they can finish the job in:

Correct — B.
The key reads the stem as A doing 3 × B's work per day.
Time is inverse to efficiency: if A takes t days, B takes 3t.
Gap: 3t − t = 60, so t = 30. A takes 30 days, B takes 90.

Combined rate: 1⁄30 + 1⁄90 = 3⁄90 + 1⁄90 = 4⁄90 of the job per day
Together: 90 ÷ 4 = 22.5 days → option (b)

Why the others are wrong

• (a)
  19.2 days — 19.2 days fits neither reading of the ratio. Three times as efficient gives 22.5 days, and four times as efficient gives 16 days.
• (c)
  16 days — 16 days is what you get if "three times more" means four times as efficient: A 20 days, B 80 days, together 80 ÷ 5 = 16. The key does not take that reading.
• (d)
  20 days — 20 days is A's time alone under the four-times reading, not the pair's time. Under the keyed reading A alone needs 30 days and the two together 22.5.

For a fixed job, efficiency and time are inversely proportional. If A does 3 times B's daily work, A needs one-third of B's time, so the times are t and 3t.

The 60-day gap is the difference of those times, 2t = 60. Once each time is known, add the daily rates, never the days, and invert the total.

Key facts

• Efficiency ratio 3 : 1 means time ratio 1 : 3 for the same job.
• Two workers together: time = product ÷ sum of their times, 30 × 90 ÷ 120 = 22.5 days.
• Rates add, times do not: 1⁄30 + 1⁄90 = 4⁄90 of the job per day.

Common traps

• Giving the more efficient worker the longer time: efficiency 3 : 1 means time 1 : 3
• Averaging or adding the two times (30 and 90) instead of adding the daily rates

No directly related past PYQ was found.