A does 75% of a work in 50 days. He then calls in B and they together finish the remaining work in 10 days. How long B alone would take to complete the whole work?

Let total work to be done = 200 units

Work done by A in 50 days = $$\frac{75}{100} \times 200 = 150$$ units

A's efficiency = $$\frac{150}{50} = 3$$ units/day

Remaining work = 200 - 150 = 50 units

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

Now, A and B complete remaining work in 10 days

=> $$(3 + x) \times 10 = 50$$

=> $$3 + x = \frac{50}{10} = 5$$

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

$$\therefore$$ Time taken by B to complete the whole work alone = $$\frac{200}{2} = 100$$ days

=> Ans - (C)