Hello!
As for algebra, as Thalia said, numbers are variables. The only thing: In any algebraic expression/equation, one letter cannot equal the other. If A=5, O cannot be 5. For example:
2x + 5 = 19
x can be anything; however, from the problem, you could figure it out. Basically letters are equal to numbers in a certain equation. In equation 1, A could equal 5, but in another, A can equal 3. Was this helpful or did I confuse you more? Give a certain problem, so we can help you out.