In algebra, letters are used when numbers are not known. Algebraic terms, such as \(2s\) or \(8y\), leave the multiplication signs out. So rather than \(2 \times s\), write \(2s\), and rather than \(8 ...

Substitution means putting numbers in place of letters to calculate the value of an expression. An expression can be a combination of numbers, letters and operators, grouped together.