arithmetic - 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 - skillgun

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