Delhi Police Constable Exam Model Paper 4

Given:
A pipe can fill a tank in 8 hours. After half of the tank is filled, 3 more similar pipes are opened. Calculation:
Let efficiency of 1 pipe be 1 unit/hr
Total capacity of tank = 1 × 8 = 8 units
Pipe 1 can fill a tank in 8 hours
⇒ Pipe 1 can fill half of the tank in 8/2 = 4 hours.
After half time, Number Of pipes $=1+3=4$
Empty tank = Half of initial capacity $=\frac{8}{2}=4$ units
Now, time taken to feel remaining half tank = 4/4 = 1 hr
∴ Total time taken to fill the tank completely $=4+1=5$ hours.