Addition and Multiplication are very similar. Multiplication is just repeated addition. If you do 3*2 its just adding 3, 2 times to each-other. The same goes for division. Its repeated subtraction. 15/3 If you subtract 3 from 15 how many times can you do it. There are also differences though. Multiplication is usually used to calculate different things though. It can calculate area and volume while addition usually doesn't. Division is used to find how much of something can fit into something. But subtraction usually is only used for subtracting other things. Those are similar things and different things for this.
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.
