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