The owner of a bagel store collected the following data about the number of bagels sold:
Plain- 9
Cheese-6
Cinnamon-5
Predict how many cheese bagels will be sold if 300 customers are served?

I reasoned the answer out to be 90, since it doesn't tell you the length of time or number of customers from which the data was collected- I assumed from the data that he normally sold 20 a day and 6 of those were cheese, so... 6/20 which is 3/10 and 3/10 of 300 is 90. Teacher said it was wrong... what am I missing here?

I don't want to say that your teacher is wrong but, adding all the bagels together gives you 20. 6 out of 20 gives you 30% of total bagels sold. 30% of 300 is 90, like you said.