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Pipe P alone can fill the tank in 20 hours while Pipe Q alone can fill the tank in 30 hours. Pipe R alone can empty the tank in 60 hours. If pipe P and Q open till half of the tank gets filled then all three pipes are opened to completely fill the tank. Total time required to fill the tank is –


1. 18 hours
2. 10 hours
3. 15 hours
4. 13.5 hours
5. 12 hours

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Best answer
Correct Answer - Option 4 : 13.5 hours

Given:

Pipe P's 1 hour's work = 1/20

Pipe Q's 1 hour's work = 1/30

Pipe R's 1 hour's work = 1/60

Pipe (P + Q)'s 1 hour's work = 1/20 + 1/30 = 1/12

Pipe(P + Q - R)'s 1 hour's work = 1/20 + 1/30 - 1/60 = 1/15

Time taken by Pipe P and Q to fill half of the tank = 1/2 × 12 = 6 hours

Time taken by three pipes to fill other half of the tank = 1/2 × 15 = 7.5 hours

∴ Total time required to fill tank = 6 + 7.5 = 13.5 hours

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