Painter 'A' can paint a house in 10 days and 'B' can do it in 20 days. With help of 'C', they did the job in 5 days only. Then, 'C' alone can do the job in:

Let total work to be done = 100 units

Painter 'A' can paint a house in 10 days and 'B' can do it in 20 days

=> A's efficiency = $$\frac{100}{10} = 10$$ units/day

B's efficiency = $$\frac{100}{20} = 5$$ units/day

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

According to ques, all of them complete the job in 5 days

=> $$(10+5+x) \times 5 = 100$$

=> $$x+15=\frac{100}{5}=20$$

=> $$x=20-15=5$$

$$\therefore$$ Time taken by C alone to do the job = $$\frac{100}{5}=20$$ days

=> Ans - (A)