It's 35.
If it's odd then it's the number in the middle. If it's even, then take the two numbers it's between, add, then divide by 2. If it's hard to find, then do a / on either side until you get to the middle. Like yours, if it's even, then add the two, 31 + 39, then divide the sum by 2. 31 + 39 = 70/2 = 35
35 = Median.