Question

Which number should replace the question mark?

Answer 1

13
Starting from the top left going clockwise among the wheels and inside the wheels themselves we can see the difference in the numbers start with 2 then 3 then 4 and finally 5. So in the third wheel going clockwise from 1; 1 + 4 => 5; 5 + 4 => 9; 9 + 4 => 13.