Every week it grows 7 total inches. So, "x" stands for week. The equation "1x = 7" means 1 week equals 7 inches. This is the simplest equation to write this problem.
The average is (time)/(number of problems)=(45/60)/20=(3/4)/20=3/80 hours per problem. In minutes, the answer would be 3/80*60=9/4 minutes per problem.