Using letters to denote numbers may look like just a simple trick of notation, but it is the fundamental idea of algebra. *Abstraction* allows us to do general computations, not just arithmetic calculations with actual quantities.
Here is a handout for the first class of an Algebra/Pre-Calculus class.
This also shows that if someone is able to solve an equation (no matter how simple), then he/she has already obtained the key skill needed for computer programming.