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 = 1/15 .

Therefore B's 1 day's work = ( 1/15 - 1/25) = 4/150 = 2/75 .

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

__Answer with 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 = 1/15 .

Therefore B's 1 day's work = ( 1/15 - 1/25) = 4/150 = 2/75 .

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