Chapter Overview



Substituting numbers into an expression

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Substituting numbers into an expression

​​In a nutshell

Substitution means replacing the variable with a number. Substitution helps find the value of simple expressions and formulae. You should be able to substitute positive or negative numbers. Take care when substituting in for more than one variable or with powers.

Recap on negative numbers

+×+=++ \times + = +​​
+×=+ \times - = -​​
×+=- \times + = -​​
×=+- \times - = +​​


Substitute the value into the expression and then calculate the answer.

Example 1

Find the value of 6n+76n+7 when n=8n=8.

6n+7=6×8+7=55\begin {aligned}6n+7 &= 6 \times 8 + 7 \\ &=\underline {55} \\\end {aligned}​​

Example 2

Find the value of 6n+76n+7 when n=2n=-2.

6n+7=6×2+7=5\begin {aligned}6n+7 &= 6 \times -2 + 7 \\ &= \underline{-5} \\\end {aligned}​​

Example 3

Find the value of abc\frac {ab} c when a=15,b=4,c=3a=15, b=4, c=3.

abc=15×43=20\begin {aligned}\frac {ab} c &= \frac {15 \times 4} 3 \\ &= \underline{20} \\\end {aligned}​​

Example 4
Maths; Formulae and equations; KS3 Year 7; Substituting numbers into an expression

The volume of the cuboid is given by V=LwhV = Lwh. Find the volume when L=5,w=2,h=3L=5, w=2, h=3.

V=L×w×h=5×2×3=30\begin {aligned}V &= L \times w \times h \\& = 5 \times 2 \times 3 \\&= \underline{30} \end {aligned}​​

Frequently Asked Questions (FAQ)


  • Question: What are the rule for multiplying negative numbers?

    Answer: When multiplying two numbers, if one of the numbers is positive and one is negative, then the answer will be negative. If both numbers are negative, then the answer will be positive.

  • Question: What is substitution?

    Answer: Substitution means replacing. In algebra we replace the variable (or letter) with a number.

  • Question: What is substituting into an expression?

    Answer: Substitution means replacing the variables in an algebraic expression with their numerical values. We can then work out the value of the expression.



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