If 15 men take 21 days of 8 hours each to do a piece of work, then what is the number of days of 6 hours each that 21 women would take, if 3 women would do as much work as 2 men ?

A. 18

B. 20

C. 25

D. 30

Find the no. of man-hrs to complete the job
15 * 21 * 8 = 2520 man hrs

2 man-hrs = 3 women-hrs 
therefore
1 women-hr = 2/3 man-hr

let d = no. days for these women to do the job
21 * 2/3 * 6d = 2520
84d = 2520
d = 30 days