Let's look at the changes between each of the numbers.
60 - 56 = 4
56 - 48 = 8
48 - _ = ?
_ - 20 = ?
So we have a sequence:
4, 8, ?, ?
Let's start with something simple. Continue that sequence like this:
4, 8, 12, 16
Now try that with the original numbers:
60 - 56 = 4 (good)
56 - 8 = 48 (good)
48 - 12 = 36 (maybe)
36 - 16 = 20 (good)
So the sequence subtracting 4, then 8, then 12, then 16, and so on works.
The missing number should be 36.