A and B are two inlet pipes A can fill 4/5 part in 16 min. and B can fill 3/4 part in 15 min. If both are opened how long it will take to fill the tank.

$A\to {\frac{4}{5}}^{th}$ part in 16 min

1 min $=\frac{4}{5\times 16}={\frac{1}{20}}^{th}$

$B\to {\frac{3}{4}}^{th}$ part in 15 min

1 min $=\frac{3}{4\times 15}={\frac{1}{20}}^{th}$

Both are opened simult ancously then

$=\frac{1}{20}+\frac{1}{20}=\frac{2}{20}={\frac{1}{10}}^{th}$

$\Rightarrow$ 10 min are needed to completely fill the tank.