A man completes 7/8 of a job in 21 days. How many more days will it take him to finish the job if quantum of work is further increased by 50% ?

Correct Answer: (D) 15

The man completes $\frac{7}{8}$$\frac{7}{8}$ of the job in 21 days, so his working rate is $\frac{7\mathrm{/}8}{21}=\frac{1}{24}$$\frac{7/8}{21}$ = $\frac{1}{24}$ of the job per day. The remaining work of the original job is $1 -$$\frac{7}{8}$ = $\frac{1}{8}$. Increasing the total quantum of work by 50% adds an extra $\frac{1}{2}$$\frac{1}{2}$ unit of work. Thus, the total remaining work to be completed is $\frac{1}{8}+\frac{1}{2}=\frac{5}{8}$$\frac{1}{8}$ + $\frac{1}{2}$ = $\frac{5}{8}$. The additional days required are $\frac{5\mathrm{/}8}{1\mathrm{/}24}=\frac{5}{8}\times 24=15\text{ days}$$\frac{5/8}{1/24}$ = $\frac{5}{8}$ $\times$ 24 = 15$\text{ days}$.