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

so, $\frac{120\times 80}{120-80}=\frac{9600}{40}=240 \min$

Hence, full tank will be filled in 240 min when both the pipes are opened simultaneously.

But the question is to calculate the time taken to fill $\frac{3}{4}$^th tank.

So, $240\times \frac{3}{4}=180 \min$

Hence option B is correct.