A can do work alone in 15 days whereas B can destroy entire work in 20 days. They are working on alternate days with a working on the first day, B working on the second day, then how many days will work completed?

Answer 1

on any two consecutive days,

1/15 + 1/20 = 7/60 of the job is done
So, after 8 pairs (16) days, 56/60 = 14/15 of the job is done

A finishes the job on day 17.