BPSC 65th Prelims Exam 15 Oct 2019 Solved Paper

Net work if both tap is open for one hour $=\frac{1}{3}-\frac{1}{4}=\frac{1}{12}$
$\therefore$ Total time needed to fill the water tank when both the taps are opened together
$=\frac{1}{\frac{1}{12}}=12$ hours
Since $\frac{1}{3}\mathit{r}\mathit{d}$ is full so remaining is $1-\frac{1}{3}=\frac{2}{3}$
Now, Time taken to fill the $\frac{2}{3}\mathit{r}\mathit{d}$ remaining part of water tank