Technically, the problem as stated is invalid; ANY next number would form a valid sequence.
However, there is a clear pattern in the numbers shown: the differences between successive term from an arithmetic sequence.
That means the differences between those differences (mathematically, the second differences of the given sequence) are constant; in fact they are all -2.
We can display all of this in an array like this:
20 27 32 35 36 the given terms of the sequence
7 5 3 1 the first differences
-2 -2 -2 the second differences
A constant second difference means the sequence can be formed by a polynomial of degree 2:
Use that general quadratic equation with the fact that t(1)=20, t(2)=27, and t(3)=32 to write and solve a system of three equations in a, b, and c.
That will give you what you are looking for -- a quadratic equation that determines the n-th term in the sequence.
