After working for 15 days Adam realises that only 60% of the work is over and he employs Pat who is half as efficient as Adam to help him. When will the task be completed?

Explanation:

Adam alone finishes 3\5th of the work in 15 days.

He can finish the whole work in 25 days.

Therefore, Pat alone can finish the entire work in 2\5×2 = 50 days [As Pat is half as efficient as Adam]

Or Adam and Pat working together will complete the remaining 2\5th of the work in 2\5[25×50\25+50]=20\3 = 6.67 days

Therefore, the whole work can be completed in 15 + 6.67 = 21.67 days