At 6:00am, the temperature was 75 degrees. By noon, it was 18 degrees warmer. A storm rolled through at 4:00pm and dropped the temperature 21 degrees. What was the temperature at 4:00pm?

At 4:00pm the temperature is 72 degrees

First you know that at 6 it was 75 degrees. Now it's noon and the temperature is 18 degrees warmer. So when you add 75 and 18 you get 93 degrees. When the storm rolls in at 4 it says that your temperature dropped 21 degrees. So you need to subtract 21 from 93 (93-21) which will get you at 72 degrees at 4pm