Median means you have to take the middle of the set of data
Step 1: Put the numbers in order from smallest to largest
12 15 15 15 17 17 18 20 21
Step 2: To find the exact middle each "level" take away the 1st and last number <<<Does that make sense?
12 15 15 15 17 17 18 20 21
15 15 15 17 17 18 20
15 15 17 17 18
15 17 17
17
The median is 17
Hope this helped!