If 5 is transformed into 11, and 12 is transformed into 25, then what does 15 become?

If 5 is transformed into 11, and 12 is transformed into 25, then what does 15 become?

Taking a look at potential patterns, we see:
5 * 2 + 1 = 11
12 * 2 + 1 = 25

Using this formula, we have:
15 * 2 + 1 =31