A does 75% of a work in 25 days. He then calls in 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 25 days = $$\frac{75}{100} \times 100 = 75$$ units

A's efficiency = $$\frac{75}{25} = 3$$ units/day

Remaining work = 100 - 75 = 25 units

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

Now, A and B complete remaining work in 5 days

=> $$(3 + x) \times 5 = 25$$

=> $$3 + x = \frac{25}{5} = 5$$

=> $$x = 5 - 3 = 2$$

$$\therefore$$ Time taken by B to complete the whole work alone = $$\frac{100}{2} = 50$$ days

=> Ans - (A)