I need a formula or code to calculate the following:

I know that it takes 2 hours to complete 25 questions. I am trying to
calculate how long it takes to do 1 question, then multiply that by however
many questions and the result should be to the nearest 15 minutes in decimal
format.

Example: If 1 question is 4.8 minutes: then 10 questions would be 48
minutes. I want it to round to 45 minutes and give the result .75.....If 5
quesitons is 24 minutes, then I want it to round to .50 minutes.