Alrighty, so, I believe the answer you're looking for is 13.
First you find the median which is 25.5
Next you find the median between the numbers 3, 16, 18, and 25 which is 17.
Next you find the median between the numbers 26, 29, 31, and 50 which is 30.
Subtract. 30-17=13
It's been a while since I've done this so I hope this is correct! (:
