Pipe D1 can fill an empty tank in 80 minutes and Pipe D2 can empty the same tank in 100 minutes. If pipes are opened simultaneously, then in how much time the tank gets completely filled ?

A/c to question ,

$$ \frac{1}{80} - \frac{1}{100} = \frac{5}{400} - \frac{4}{400} = \frac{1}{400}$$

If pipes are opened simultaneously, then the tank will be filled in 400 minutes.

So, the answer would be option b)400 minutes

OR

Let's assume the capacity of tank is 400 units.

Efficiency of D1 = $$\frac{400}{80}$$ = 5 units/minute

Efficiency of D2 = - $$\frac{400}{100}$$ = - 4 units/minute [here negative sign represents that pipe is emptying the tank.]

If pipes are opened simultaneously, then the tank will be filled = $$\frac{400}{5-4}$$ = 400 minutes