10 men or 20 boys can finish a piece of work in 15 days. How many boys are required in place of 8 men to complete the same work?

16

10M 0r 20B can complete a work in 15 days i.e. same amount of work is done by 10M  as  done by 20B in same number of days .

so ratio of  efficiency   M    :    B

                                 2    :    1

that means 1M = 2B

8M = 8 × 2B = 16B

so 16 boys can do same work as done by 8 men