A tanker can be filled by two pipes A and B in 20 minutes and 30 minutes respectively. How many minutes will it take to fill the tanker from empty state if A is used for half the time and A and B fill it together for the other half ?
Solution: Part filled by (A+B) in 1 min. = (1/20 + 1/30) = 1/12.
Suppose the tank is filled in x min.
Then, x/2(1/12 + 1/20) = 1
=> x/2 * 8/60 = 1
=> x = 120/8 = 15 min
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