25 men reap a field in 20 days . when should 15 men leave the work

25 men reap a field in 20 days . when should 15 men leave the work.if the whole field is to be reaped in 37-1/2 days after they leave the work?
A 5 days
B 10 days










10 workers work for 75/2 days & 25 workers work for x days:
x/20 + 75/2 (1/50) = 1
Ans : A