arithmetic - a large tanker can be filled by two pipes a and b in 60 minutes and 40 minutes respectively how many minutes will it take to fill the tanker from empty state if b is used for half the time and a and b fill it together for the other half - skillgun

Part filled by (A + B) in 1 minute = 1/60 + 1/40 = 1 24
Suppose the tank is filled in x minutes.
Then, x/2(1/24 + 1/40) = 1
=>x/2 x 1/15 = 1
=> x = 30 min.