How do u answer this question ... 18+13=

18+13 is 31.

because 18+13 can be

10 + 8 + 10 + 3

and if you add 8 and 3, it's 11

if you add 10 with 10, it's 20

so then if you add 11 with 20, it's 31

there are many different ways to add numbers, but this is just one of them