Expain the diffents between numerical expressions and algebraic expressions

Numerical expressions are numbers and signs, like below. The following are some examples of numerical expressions, numerical expressions do not use letters. 4 + 20 - 7, (2 + 3) - 7, (6 * 2) : 20, 5 : (20 * 3) An algebraic expression uses letters and it says you to find "x/y/z". Example; 4+20xy-7x, (2x+3) - 7y+20.