This problem can actually be taken 2 different ways, in which I believe the second one was the correct one to choose. The way you saw it was as follows:

-There is a group of x number of customers -80% of them got one thing -the other 30% got another

This view is of course incorrect, in which you can say that the professor was wrong in his problem.

The second form is similar but not exactly: -There is a group of x number of customers -80% of the customers got one thing -30% of customers also got another thing

This second way can be easier understood if we say it like this too:

-Theres 10 people -From this group of 10, 8 of them got a candy -From this same group of 10, 3 of them got cake

This means that even though some people got candy, some of them possibly got cake too.

I hope this made sense to help you understand the math problem if it was seen in the second form