121. A does 80% of a work in 20 days. He then calls in B and they together finish the remaining work in 3 days. How long B alone would take to do the whole work?

✅ The correct answer is option C.
Whole work is done by A in

20 x
5

= 25 days.

4

Now,

1 –
4

i.e.,
1
work is done by A and B in 3 days.

5
5

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

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

25
15

B’s 1 day’s work =

1
–
1

=
4
=
2
.

15
25
150
75

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

2
2