Question

A and B can do a piece of work in 12 days. They started to work together but A left after a few days. If B alone worked for 20 days, the whole work was finished in 24 days. How long would A take to complete the work???

question is really confusing :/

Answer 1

1/a + 1/b = 1/12

So, if A left after n days, A&B did n/12 of the work.

20/b = 1 - n/12
240 = 12b - nb
b = 240/(12-n)

so,
1/a + (12-n)/240 = 1/12
240 + (12-n)a = 20a
a = 240/(8+n)

I agree the question is confusing. The exact conditions seem ambiguous.