Question 253212
A "tricky" question but not a "trick" question!
Let's first find the fill rate and emptying rate.
Notice that the tank is being emptied (30 mins.) faster than it is being filled (45 mins.).
If the tank can be filled in 45 minutes, then {{{1/45}}} of the tank can be filled in 1 minute.
If the tank can be emptied in 30 minutes, then {{{1/30}}} of the tank can be emptied in 1 minute.
If both events are occuring simultaneously, then the net result is:
{{{1/45-1/30 = (2-3)/90}}}={{{-1/90}}} This means that {{{1/90}}} of the tank will be emptied (negative) in 1 minute, so it will take 90 minutes to empty the tank if both pipes are open.