Rico was born 6 years after Nico. The sum of their age is 36. How old is Nico?

Rico was born 6 years after Nico. The sum of their age is 36. How old is Nico?

Let Rico's age be r
Let Nico's age be n

We're given two equations:

1. r = n + 6
2. n + r = 36

We plug equation (1) into equation (2) for r:
n + n + 6 = 36

To solve this equation for n, we type it in our search engine and we get:
n = 15