2nd way: List the numbers from greatest to least.
21,17,13,11,10,8,6,4,3. The half way point to 9 numbers is the fifth number which is 10.

3rd way: List the numbers from least to greatest. Then cross out one number from the front of the list and one from the back, repeat until you have the middle number. 3,4,6,8,10,11,13,17,21