# P and Q can fill a water tank in 9hr and 12 hr respectively but R can

| P and Q can fill a water tank in 9hr and 12 hr respectively but R can empty it in 18 hr. Pipes P, Q and R start at 2 PM, 3PM and 4 PM respectively. At what time tank will be completely filled.

A. 7 PM

B. 8 PM

C. 9 PM

D. 10 PM

### Right Answer is: C

#### SOLUTION

Total capacity of tank = 36 units

Net efficiency of P, Q and R = 4+3 -2 = 5

Pipe P starts at 2 PM.

Tank filled by pipe P in 2 hrs = units

Pipe Q starts at 3 PM.

Water tank filled by pipe Q in 1 hr = units

Total water tank filled in 2 hr = 8 +3 = 11 units

Empty tank = 36 units – 11 units = 25 units

Time taken to fill the empty tank =

Hence tank will be completely filled at 9 PM

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P and Q can fill a water tank in 9hr and 12 hr respectively but R can