15 men can build a 500m long wall in 30 days. In how many days can 30 men build a 1.2 km long wall?

A. 48 days
B. 24 days
C. 40 days
D. 36 days

No. of days is
directly proportional to the length of the wall and
inversely proportional to the number of men No. of days = (1200/500)*(15/30)*30 = 36 days