1 tap can fill a tank in 9 hours, a second in 12 hours alone,what fraction will be filled in 3 hours if both taps are open together

Answer 1

in one hour, the taps can fill

1/9 + 1/12 = 7/36 of the tank.

So, in 3 hours, they fill 3 times that much:

3*7/36 = 7/12