A can do a piece of work in 20 days, which B can do in 12 days. B worked at it for 9 days. A can finish the remaining work in

work done by $\mathrm{B}$ in 9 days $=\frac{9}{12}=\frac{3}{4}$ Remaining work $=1-\frac{3}{4}=\frac{1}{4}$ $\therefore$ Time taken by $\mathrm{A}=\frac{1}{4} \times 20=5$ days Alternative:

Work done by B in 9 days

= 9 × 5 = 45

Remaining work = 60 – 45 = 15

Time taken by A to complete the remaining work

$=\frac{15}{3}=5$ days