The 3rd term in a geometric sequence is 40. The 5th term is 10. What are the possible values of the 4th term?

To answer, we let x be the 4th term. We use the concept of geometric mean in order to answer this question. The ratio of the fourth term and the third term is equal to the ratio of the fifth term and the fourth term. That is,

x/40 = 10/x

The values of x from the equation are 20 and - 20.