I know the method
7+6 = 113
the method:
1. substract two number on the left then you will get the first digit of the answer : 7-6= 1
2. add up two numbers on the left which is : 7+6 = 13
3. combine numbers we got from that 2 steps. 1 and 13 -> 113
so, the question of the riddle is:
18+13 = ?
the step
1. 18-13 = 5
2. 18+13 = 31
3. So the answer is 531
thanks
regards, belpas