Multiple choice

A can do a piece of work in 20 days and B can do same work in 30 days. They started the work together but after 9 days A left the job. In how many days B will complete the rest of the work

  1. 7 1/2 days

  2. 12 1/2 days

  3. 15

  4. 20

  5. None of these
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A and B together do (1/20 + 1/30) = 5/60 = 1/12 of the work per day. In 9 days, they complete 9/12 = 3/4 of the work, leaving 1/4 of the work for B. Since B completes 1/30 of the work per day, B takes (1/4) / (1/30) = 30/4 = 7.5 days.

AI explanation

Assuming the total work is the least common multiple of 20 and 30, we get 60 units. The daily work rates of A and B are 3 units and 2 units respectively, so working together for 9 days they complete 45 units. This leaves 15 units of work for B to complete alone. Dividing the remaining 15 units by B's daily rate of 2 units gives 7.5 days. The result is 7 1/2 days.

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