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Level 3-4 Algebra - Putting Numbers in Place of Letters

Practise substituting numbers for letters in KS3 algebra. Replace variables carefully, use brackets, and follow order of operations to evaluate expressions correctly.

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Fascinating Fact:

When there are two letters, such as 2a + 3b, you replace each one separately; for example, if a = 4 and b = 2, the answer is 14.

In KS3 Maths, substitution means putting numbers in place of letters to find values. Work with one variable at a time, use brackets for negatives or products, and follow BODMAS/BIDMAS to calculate correctly.

Key Terms

Variable: A letter that stands for a number, such as a, b or x.
Substitution: Replacing each variable with a given number before calculating.
Order of operations: The agreed order (brackets, indices, multiply/divide, add/subtract) used to evaluate expressions.

Frequently Asked Questions (Click to see answers)

How do you substitute into 2a + 3 when a = 5?

Replace a with 5: 2 × 5 + 3 = 10 + 3 = 13. Always multiply before adding, following the order of operations.

Do I need brackets when substituting negative numbers?

Yes. Use brackets to avoid mistakes. For a = −3 in 2a², write 2(−3)² = 2 × 9 = 18.

How do I substitute when there are two letters like 2a + 3b?

Replace each letter with its value, then calculate. If a = 4 and b = 2, 2a + 3b = 2×4 + 3×2 = 8 + 6 = 14.

Try These Related Quizzes

1 .

If a = 4 and b = 2, what is the value of a² / b?

16 / 2 = 8

2 .

If a = 2, b = 3, c = 4 and d = 5, what is the value of 2a + 3b + c² - d

4 + 9 + 16 - 5 = 24. Don't forget 'IF IN DOUBT, WRITE IT DOWN AND WORK IT OUT!

3 .

If a = 6 and x = 8, which of the following gives a value of 22?

a + x²

a + 2x

2a + 2x

2ax

'If in doubt, write it down and work it out'

4 .

If a = 2 and b = 3, what is the value of a + b + a?

2 + 3 + 2 = 7

5 .

If x = 6 and y = 7, which of the following gives a value of 55

x² + y²

xy²

x + y²

2x + 2y

You can make an estimate before you find the right answer. x² + y² would be too high, as would xy². You can then check the other two more likely answers

6 .

If a = 4 and b = 8, what is the value of 2a + x²

It is impossible to say

Notice that we did not give you a value for x and therefore you cannot work out the expression!

7 .

If x = 9 and y = 23, what is the value of y - x?

23 - 9 = 14

8 .

If x = 5 and y = 10, what is the value of xy / 10

Two letters together need to be multiplied together

9 .

If a = 4 and b = 5, what is the value of 2a + b