Question

What number do the three letters O, H and N represent in order to make this addition problem correct?

  O H
  O H
  O H
+ O H
 ______
  N O

Answer 1

O can be either 1 or 2 because if O is greater than 2 than the answer would be a three digit number

4×H has to be a even number so O has to be 2 not 1. Only if H=3 we can have O=2.
Therefore the numbers are

23
23
23
23
92

ie., O = 2, H = 3 & N = 9.