A does 60% of a work in 15 days. He then calls B, and they together finish the remaining work in 5 days. How long B alone would take to do the whole work?

Let total work to be done = 100 units

Work done by A in 15 days = $$\frac{60}{100} \times 100 = 60$$ units

A's efficiency = $$\frac{60}{15} = 4$$ units/day

Remaining work = 100 - 60 = 40 units

Let B's efficiency = $$x$$ units/day

Now, A and B complete remaining work in 5 days

=> $$(4 + x) \times 5 = 40$$

=> $$4 + x = \frac{40}{5} = 8$$

=> $$x = 8 - 4 = 4$$

$$\therefore$$ Time taken by B to complete the whole work alone = $$\frac{100}{4} = 25$$ days

=> Ans - (A)