Question

Riley is 3 years older than his sister. Which of the following expressions shows the sum of their ages if s represents Riley's sister's age?

3s
s + 3
2(s + 3)
2s + 3

Answer 1

2s+3 because Riley is represented by 3+s and the other s represents Riley's sister. 

Answer 2

2s+3 Why? because Riley is s+3 (His sister's age + 3). If we add them together we have Riley (s+3) and sister (s) which is (s+3)+(s). That is the same as s+s+3 which you can write as 2s+3