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 $\frac{\mathrm{xy}}{\mathrm{x}+\mathrm{y}}$

so, $\frac{10\times 15}{10+15}=\frac{150}{25}=6 \mathrm{hours}$ 

Both the pipes together can fill the tank in 6 hours.

Hence option A is correct