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, $\frac{18\times 12}{12+18}=\frac{216}{30}=\frac{36}{5}=7\frac{1}{5} \mathrm{hours}$

When both the pipes are opened simultaneously. tank will be filled in $7\frac{1}{5}$ hours

Hence option A is correct