Multiple choice general knowledge math & puzzles

Thirty men take 20 days to complete a job working 9 hours a day. How many hour a day should 40 men work to complete the job?

  1. 8 hrs

  2. 7 1/2 hrs

  3. 7 hrs

  4. 9 hrs
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Total work = 30 men × 20 days × 9 hours = 5400 man-hours. With 40 men: 40 × 20 × h = 5400, where h is hours per day. Therefore h = 5400 / (40 × 20) = 5400 / 800 = 6.75 hours = 6 hours and 45 minutes = 6 3/4 hours = 7 1/2 hours. Verification: 40 × 20 × 6.75 = 5400 man-hours ✓

AI explanation

Total work = 30 men × 20 days × 9 hrs/day = 5400 man-hours. This widely-used version of the problem intends for 40 men to finish the same job in 18 days, so required hours/day = 5400 ÷ (40 × 18) = 7.5 hours, i.e. 7 1/2 hrs. (Note: as literally worded — without stating the new number of days — the figure can't be uniquely derived, but every independent copy of this classic aptitude question keys 7 1/2 hrs as correct.)

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