Can you explain how to solve this math problem: Jim decides he wants to practice for a marathon coming up.  He starts off by running 5 miles every day for 30 days straight.  He then pushes himself to run 10 miles a day for the next 30 days.  Finally, as marathon day gets closer Jim runs 20 miles a day for 30 days straight.  How many miles does Jim run in total during the 90 days?