Multiple choice

A and B, working separately, can do a piece of work in 9 and 15 days, respectively. If A starts the work and both of them work for a day alternately, then the work will be completed in

  1. 10 days

  2. 11 days

  3. 9 days

  4. 12 days
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

A does 1/9 per day, B does 1/15. In 2 days, they do 1/9 + 1/15 = 8/45. In 10 days (5 cycles), they do 40/45. On the 11th day, A does 1/9 = 5/45. Total = 45/45. Work finished in 11 days.

AI explanation

The total work is the least common multiple of 9 and 15, which equals 45 units. A's rate is 5 units per day and B's rate is 3 units per day. Every two days they complete 8 units, so in 10 days they complete 40 units. This leaves 5 units for the eleventh day, which A completes alone, making the total time 11 days.

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