If 1 plus 4 = 5, 2+5 = 12, 3 plus 6=21 what does 5+8= ? i think 47

1 + 4 = 5, in this question, there is 1 "4", and 1 + 4 = 5

2 + 5 = 12, in this question, there are 2 "5", and 2 + 5 + 5  = 12

3 + 6 = 21, in this question, there are 3 "6", and 3 + 6 + 6 + 6 = 21

So for 5 + 8, there are 5 "8", so:

5 + 8 + 8 + 8 + 8 + 8 = 45

45 is your answer

hope this helps