# Help with a puzzle from an old book

I found a very nice puzzle from a old book written in Spanish. I got stuck after a few hours, the only possible solution I found was that the numbers inside the circle have a kind of succession. For instance: We have 20 in the first circle, then 15 and 16. So, I thought that 20 and 16 are divisors of 4, and the number is decreasing from 20 to 16. For this reason, I can think that for 15, the next value is gonna be 12. In conclusion, the letter B (Not E because the first number start from the square of number 1).

But I don't think that my solution could be true.

Do you have any ideas or hints to solve this puzzle? Or maybe you think that my solution was correct?

The answer could be:

E

The reason for that is:

The center number rotates clockwise from 20 to 15 to 16. To get from the previous number to the next number, subtract the product of the 2 lower numbers and add the product of the 2 upper numbers
E.g. 20 + 1 * 1 - 3 * 2 = 15
15 + 3 * 1 - 2 * 1 = 16
Hence, following the pattern, the next number should be 16 + 2 * 3 - 1 * 1 = 21. Following the same clockwise rotation of the center number results in option E.