Your equation would be division. 4 1/2 divided by 1 1/2 would give you how many lessons could be completed in a night. 4 1/2 divided by 1 1/2 would be 3. You can check it by multiplying 1 1/2 by 3. 3 is the number of classes and 1 1/2 is how long each lesson is. Since 1 1/2 times 3 is 4 1/2, it would take 4 1/2 hours to complete 3 lessons.
