You can cancel out units just the same as you can cancel down numbers (in fractions)
(90 feetcubed divide by minutes ) * 45minutes/1 = 90*45 feetcubes (the minutes cancel out)
Or you can just think logically
1 cubic ft per minute for 45 minutes = 1*45cubicfeet
2 cubic ft per minute for 45 minutes = 2*45cubicfeet
...
90 cubic ft per minute for 45 minutes = 90*45cubicfeet
+118654 
