Lin runs 4 laps in 5 minutes. You must turn this into a unit rate. In order to do this, you divide 4 laps by 4 to get 1 lap. To get the equivalent, you also have to divide 5 minutes (300 seconds) by 4. This will get you one lap for every 75 seconds. Because Lin ran 21 laps, see how many times 4 can go into 20 using a remainder if necessary. 4 can go into 21 5 times with a remainder of 1. 4*5=20 5*5=25. Now you add 75 seconds because you have an extra lap.
