A does 80% of a work in 20 days. He then calls in B and they together finish the remaining work in 4 days. How long B alone would take to do the whole work?

Let the total work be 100 units.
Find A's efficiency:
A completes 80% of the work (80 units) in 20 days.

A’s efficiency (work per day)=
20
80
​
=4 units/day
Analyze combined work:
The remaining work is 100%−80%=20% of the work, which is 20 units.
A and B together finish these 20 units in 4 days.

Combined efficiency (A + B)=
4
20
​
=5 units/day
Find B's efficiency:

B’s efficiency=(A + B’s efficiency)−A’s efficiency
B’s efficiency=5−4=1 unit/day
Calculate total time for B alone:

Time taken by B alone=
B’s efficiency
Total Work
​
=
1
100
​
=100 days