There are 2 poss.
a)20 is a unit of this set. Then 20 becomes automatically the median (the number of units in the set is odd
b)20 is not a unit of this set. Then we can not know the median. Because it means there are even number of element in the set and the median is the mean of the two element in the center. They may be 19 and 21 or 18 and 35.
2)insuff. No other info.
We only know that 20 is not an element of this set. So the number of elements in this set is even. Thus it is same like the possibility b) in 1.