Abul can do a work in 15 day and Babul in 20 days. If they work on it together for 4 days, then the fraction of the work that is left is -

Question: Abul can do a work in 15 days and Babul in 20 days. If they work on it together for 4 days, then the fraction of the work that is left is -

Solution:
Given, Abul can complete the work in 15 days and Babul can complete the work in 20 days.
Abul's 1 day's work = 1/15
Babul's 1 day's work = 1/20

∴ (Abul + Babul)'s 1 day's work = (1/15 + 1/20) part
= (4 + 3)/60 part
= 7/60 part

∴ (Abul + Babul)'s 4 days' work = {4 × (7/60)} part
= 28/60 part
= 7/15 part

Therefore, Remaining work = 1 - (7/15) part
= (15/15) - (7/15) part
= (15 - 7)/15 part
= 8/15 part