# 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?

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.