Question 212323
If it takes two pipes to fill the tank in 30 minutes, then the two pipelines will fill 1/30 of the tank in one minute.
If it takes one pipeline 45 minutes to fill the tank, the it will fill 1/45 of the tank in one minute.
So subtract:
{{{1/30-1/45 = 3/90-2/90}}} = {{{1/90}}}
So the other pipeline can fill 1/90 of the tank in one minute, therefore it will take 90 minutes to fill the tank.