Well, the median is the "middle" number. So first, you have to rearrange the numbers into numerical order.
10, 10, 11, 12, 15, 16, 18, 18, 18, 20
Then you cross off one from each end, until you get a final number.
You end up with 15 and 16, and 15.5 is your final answer.
Hope I helped, and feel free to contact me if you have any more questions!! :)
