16 men or 24 boys can finish a piece of work in 35 days. How many boys are required in place of 18 men to complete the same work?

27

16M or 24B can complete a work in 35 days i.e. same amount of work is done by 16M  as  done by 24B in same number of days .

so ratio of  efficiency   M    :    B

                                 3    :    2

that means 2M = 3B

18M = 9 × 3B = 27B

so 27 boys can do same work as done by 18 men