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?