You posted:
I need to display the duration of rental in number of hours and minutes as well as the cost (the rate is simply $5 for 5 or less hours, $12 for 6 or more hours, and 50 cents each hour after first 24 hours.)
Which looks like three different rates to me.

The only way to learn this, is by doing it. Take a paper and pencil, and work through some simulated rental costs, for different amounts of rental time. Do it by hand, and save your notes on the paper. That will be the calculations your program will need to complete this program.

It's not difficult to do it by hand, but it takes some time, and you have to be able to understand it. It's rare that you can code up something that works right, when you don't fully understand that something, and how it was calculated. Really REALLY rare.