What Are Variables? — Definition & Examples

Alex is doing a secret calculation. At one point, he wants to encode the equation $$2+3\cdot5=17$$ in his notebook so that only he can decipher his notes.

Alex decides to use variables $a,b,c,d$ to represent $2,3,5,17$ in some order. He then writes down the cryptic message $$ab+c = d$$ in his notebook.