there are two taps, the small one uses forty five minutes to fill a tank and the big one uses thirty minutes to do the same how many minutes will the tank be filled when the two taps were opened at the same time to fill the same tank.
rate of smaller tap = 1/45 tanks/min
rate of larger tap = 1/30 tanks/min
combined rate = (1/45 + 1/30) tanks/min
= 1/18 tanks/min
time at combined rate = 1/(1/18) min
= 18 minutes