Problem: The difference between two numbers is 13. Twice the smaller plus three times the larger is 129.

Answer: $\displaystyle 2x+3(x+13) = 129$

I have several questions breaking down this problem:

1) How do you know which one is the smaller number in the expression above? Do you just assign and compare quantities?

2) Why does it read $\displaystyle (x+13)$ when the text states thedifferencebetween two numbers is 13?

Thanks for helping me understand this better.