what number goes in the missing space?

Explanation:

Step1: Analyze the pattern in the first two circles

In the first circle, if we consider the relationship between the numbers: $4\times4 = 16$ and $4\times2=8$. In the second circle, $6\times4 = 24$ and $6\times2 = 12$. The pattern seems to be that the bottom - number is 4 times the number on the left, and the right - number is 2 times the number on the left.

Step2: Apply the pattern to the third circle

For the third circle, since the left - number is 10, and following the pattern, the right - number should be $10\times2$. $10\times2=20$

Answer:

20