15 men or 30 boys can finish a piece of work in 34 days. How many boys are required in place of  12 men to complete the same work?

24

15M or 30B can complete a work in 34 days i.e. same amount of work is done by 15M  as  done by 30B in same number of days .

so ratio of  efficiency   M    :    B

                                 2    :    1

that means 1M = 2B

12M = 12 × 2B = 24B

so 24 boys can do same work as done by 12 men