(a) 23 days

(b) 37 days

**(c) 37(1/2) days**

(d) 40 days

Explanation:

Whole work is done by A in (20 x 5/4) = 25 days.

Now, (1 – 4/5) i.e., 1/5 work is done by A and B in 3 days.

Whole work will be done by A and B in (3 x 5) = 15 days.

A’s 1 day’s work = 1/25 , (A + B)’s 1 day’s work = 1/25

B’s 1 day’s work = (1/15 – 1/15) = 4/150 = 2/75.

So, B alone would do the work in 75/2 = 37(1/2) days.