Starting at home, Luis travelled uphill to the toy store for 12 min at 15 mph. He then travels back home along the same path, downhill at a speed of 30mph. What is his average speed for the entire trip, up and back from the toy store.
I know that the amount of time downhill is 6 minutes, but I can't understand how to find the speed. There is a multiplication formula used in the final equation, and I just don't understand why the multiplication is used.