A, B and C together can complete a work in 20 days. A and B together can complete the same work in 30 days. Then C alone can complete the same work in how many days?

Answer : Option D

Explanation :

$(\mathrm{A}+\mathrm{B}+\mathrm{C})$ ’s 1 day’s work $=\frac{1}{20}$ $(\mathrm{A}+\mathrm{B})$ ’s 1 day’s work $=\frac{1}{30}$ $\therefore$ C’s 1 day’s work $=\frac{1}{20}-\frac{1}{30}=\frac{3-2}{60}=\frac{1}{60}$ $\therefore$ Required time $=60$ days. Alternative:

Efficiency of $\mathrm{C}=1$ unit

Time taken by $\mathrm{C}=\frac{60}{1}=60$ days