If 28 men can finish a work in 15 days, 21 men can finish the same work in___________?

A. 24 days
B. 14 days
C. 15 days
D. 20 days
Time and Men are inversely proportional. Time taken by 28 men to finish the work = 15 days Time taken by a man to finish the work
= 15*28 days Time taken by 21 men = (15*28)/21 = 20 days