Ok so I see where you got the 27 from because my initial thought was to multiply the 4 and 1/2 times 6 which would equal 27, but it is not asking for that.
They are saying that all six classes combined last for 4 and 1/2 hours, and that each class is the same amout of time. Which means we would have to divide 4 and 1/2 by 6 to find the answer that we are looking for. 4.5 (the decimal version of 4 and 1/2) divided by 6 equals .75. If you use the fraction and divide you would get 3/4 which is the same thing as .75.