B) 10
C) 25
D) 30
A takes 1 day to complete 1/20.
Therefore let the number of days taken to complete the whole work be x.
Therefore:
1: 1/20 :: x :20/20 ( Product of means =product of extremes)
x/20= 20/20
x=20/20*20
x=20 days
Therefore the answer = 20 days