Two pipes, A and B, can fill a tank of 7800 litres in 12 hours and 10 hours, respectively. If they are opened together, how many hours will they take to fill an empty tank of 8800 litres?​

Correct option is D

Given:

Capacity filled by A: 7800 L in 12 h

rate of A $$= \frac{7800}{12}$$​=650 L/h

Capacity filled by B: 7800 L in 10 h 

rate of B =$$\frac{7800}{10}=780$$​ L/h

Tank to be filled now: 8800 L

Formula Used:

Time = $$\dfrac{\text{Volume}}{\text{Combined rate}}$$​

Solution:
Combined rate = 650 + 780 = 1430 L/h​

​$$\text{Time}=\frac{8800}{1430}=\frac{880}{143}=\frac{80}{13}\ \text{hours}$$​