Multiple choice

A and B can do a piece of work in 30 days and 18 days respectively. A started the work alone and then after 6 days B joined him till the completion of the work. In how many days has the whole work completed?

  1. $17$
  2. $15$
  3. $9$
  4. $12$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

A's work rate is 1/30 per day. In 6 days, A completes 6/30 = 1/5 of the work. Remaining 4/5 work is done by A and B together at rate (1/30 + 1/18) = 8/90 = 4/45 per day. Time for remaining work = (4/5) / (4/45) = 9 days. Total time = 6 + 9 = 15 days.

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