(It might also have helped immensely if you had been listening
when this was being explained in class.)
The first pipe fills 1/20 of the tank each minute.
The second pipe fills 1/30 of the tank each minute.
Operating together, the two pipes fill ( 1/20 + 1/30 ) of the tank each minute.
In order to add these fractions (or any fractions), you need a common denominator.
For these particular ones, ' 60 ' is a good choice.
( 1/20 + 1/30 ) = ( 3/60 + 2/60) = 5/60 = 1/12
The rate of work is 1/12 tankful per minute .
The most sensible choice for the time is 12 minutes.
R x T = W
( 1/12 tankful per minute ) x ( 12 minutes ) = 1 full tank
