According to the formula, if pipe A can comletely fill a tank in x hours and pipe B can comletely fill a tank in y hours, then the time taken by both pipes together to fill the tank is $\frac{\mathrm{xy}}{\mathrm{x}-\mathrm{y}}$ hours.

So, $\frac{10\times 15}{15-10}=\frac{150}{5}=30 \mathrm{minutes}$

The tank will be filled by both the pipe when opened simultaneously in 30 minutes.

Hence, option C is correct