Multiple choice

P and Q working separately can do a piece of work in 10 days and 15 days respectively. If they work on alternate days beginning with P, in how many days will the work be completed?

  1. 10 days

  2. 12 days

  3. 13 days

  4. 15 days
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

P does 1/10 per day, Q does 1/15 per day. Starting with P: Day 1: 1/10 done; Day 2: 1/10+1/15 = 1/6 done; Day 3: 1/6+1/10 = 4/15 done. After 10 days (5 cycles of P+Q): 5×(1/10+1/15) = 5×1/6 = 5/6 done. Day 11 (P): 5/6+1/10 = 14/15 done. Day 12 (Q): 14/15+1/15 = 1 (complete).

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