Answer: Option D
:
D
Option a can be eliminated directly as the number of days have to be $>$ 9 days.
Answer is either b,c and d
Check for answer b.
If slower one takes 16 days, then faster takes $\frac{1}{4}-\frac{1}{16}=\frac{3}{16}\Rightarrow \frac{16}{3}$days
Slower takes $\frac{16}{2}=8$ days and faster takes$\frac{\left(\frac{16}{2}\right)}{3}=\frac{8}{3}$ days. Totally they will not take 9 days
Reverse Gear
Check for answer c
If the slower crusher takes 15 days, then the faster one takes $\frac{1}{4}-\frac{1}{15}=\frac{11}{60}\Rightarrow \frac{60}{11}$ days.
Slower does his half work in $\frac{15}{2}=7.5$days and the faster does his work in $\frac{60}{22}$. together it does not add up to 9 days. Hence, answer is option (d)