Maths · Algebra Foundations
Substitute values
Substitute numbers into expressions.
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What Does Substitution Mean?
Substitution means replacing a letter in an expression with a number. The letter is called a variable because its value can vary. When we substitute, we swap the letter for the number we're given.
Writing Multiplication Carefully
In algebra, 3n means 3 × n. We don't write the × sign to keep expressions tidy. When substituting, always put the number in brackets if it's negative or a decimal to avoid mistakes.
Order of Operations
After substituting, follow BIDMAS (Brackets, Indices, Division, Multiplication, Addition, Subtraction). Work out any powers first, then multiplication or division, and finally addition or subtraction.
Multiple Variables
Expressions can have several different letters. Substitute each variable with its given value. Make sure you replace the correct letter with the correct number every time.
Common Mistakes
Don't forget to multiply when letters sit next to numbers. Remember 5t means 5 × t, not 5 + t. Always work out powers before multiplying or adding.
Worked examples
1. Find the value of 4x + 7 when x = 3
- Substitute x = 3 into the expression: 4 × 3 + 7
- Work out 4 × 3 = 12
- Then add 7: 12 + 7 = 19
2. Find the value of 2a² when a = 5
- Substitute a = 5 into the expression: 2 × 5²
- Work out the power first: 5² = 25
- Then multiply: 2 × 25 = 50
3. Find the value of 3m - 2n when m = 4 and n = 6
- Substitute m = 4 and n = 6: 3 × 4 - 2 × 6
- Work out each multiplication: 12 - 12
- Subtract: 12 - 12 = 0
Try it yourself
Find the value of 5p + 8 when p = 2
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