To find the median of a set of numbers:
Put them in numerical order. (least to greatest, or greatest to least, either works)
Then, the number in the middle is your median.
11, 13, 17, 20, 24
The median is c. 17.
Extra:
Suppose you have an even number of data points in your set, like here:
1, 2, 3, 4, 5, 6
There's no number in the middle! So how do we get our median?
Well, if we had a middle, it would be between the 3 and 4, right?
So we meet halfway between 3 and 4 at 3½. (just an extra thing to note)