Find the missing number in the following puzzle using the diagonal sum pattern: [39, ?, 25, 13]

The numbers on the diagonal from top-left to bottom-right are 39 and 13.
Calculate their sum:

$$39 + 13 = 52$$

The numbers on the diagonal from bottom-left to top-right are 25 and the missing number (let's call it $X$).
Their sum is:

$$25 + X$$

$$25 + X = 52$$

Subtract 25 from both sides:

$$X = 52 - 25$$ $$X = 27$$

The missing number is $\text{27}$.