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Maths Quiz - Level 3-4 Algebra - Putting Numbers in Place of Letters (Questions)

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 = 2 and b = 3, what is the value of a + b + a?
[ ] 5
[ ] 6
[ ] 7
[ ] 8
2. If x = 9 and y = 23, what is the value of y - x?
[ ] 14
[ ] 16
[ ] 18
[ ] 20
3. If a = 4 and b = 5, what is the value of 2a + b
[ ] 7
[ ] 9
[ ] 11
[ ] 13
4. If x = 5 and y = 10, what is the value of xy / 10
[ ] 3
[ ] 5
[ ] 10
[ ] 20
5. If a = 6 and x = 8, which of the following gives a value of 22?
[ ] a + x2
[ ] a + 2x
[ ] 2a + 2x
[ ] 2ax
6. If x = 2 and b = 4, what is the value of x2 + b2?
[ ] 12
[ ] 16
[ ] 20
[ ] 24
7. If a = 4 and b = 2, what is the value of a2 / b?
[ ] 8
[ ] 16
[ ] 4
[ ] 2
8. If x = 6 and y = 7, which of the following gives a value of 55
[ ] x2 + y2
[ ] xy2
[ ] x + y2
[ ] 2x + 2y
9. If a = 4 and b = 8, what is the value of 2a + x2
[ ] 64
[ ] 75
[ ] 80
[ ] It is impossible to say
10. If a = 2, b = 3, c = 4 and d = 5, what is the value of 2a + 3b + c2 - d
[ ] 24
[ ] 26
[ ] 28
[ ] 30

You can find more about this topic by visiting BBC Bitesize - Writing and interpreting algebraic expressions

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Maths Quiz - Level 3-4 Algebra - Putting Numbers in Place of Letters (Answers)
1. If a = 2 and b = 3, what is the value of a + b + a?
[ ] 5
[ ] 6
[x] 7
[ ] 8
2 + 3 + 2 = 7
2. If x = 9 and y = 23, what is the value of y - x?
[x] 14
[ ] 16
[ ] 18
[ ] 20
23 - 9 = 14
3. If a = 4 and b = 5, what is the value of 2a + b
[ ] 7
[ ] 9
[ ] 11
[x] 13
Remember that when a number and a letter are together they need to be multiplied together
4. If x = 5 and y = 10, what is the value of xy / 10
[ ] 3
[x] 5
[ ] 10
[ ] 20
Two letters together need to be multiplied together
5. If a = 6 and x = 8, which of the following gives a value of 22?
[ ] a + x2
[x] a + 2x
[ ] 2a + 2x
[ ] 2ax
'If in doubt, write it down and work it out'
6. If x = 2 and b = 4, what is the value of x2 + b2?
[ ] 12
[ ] 16
[x] 20
[ ] 24
(2 x 2) + (4 x 4)
7. If a = 4 and b = 2, what is the value of a2 / b?
[x] 8
[ ] 16
[ ] 4
[ ] 2
16 / 2 = 8
8. If x = 6 and y = 7, which of the following gives a value of 55
[ ] x2 + y2
[ ] xy2
[x] 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
9. If a = 4 and b = 8, what is the value of 2a + x2
[ ] 64
[ ] 75
[ ] 80
[x] It is impossible to say
Notice that we did not give you a value for x and therefore you cannot work out the expression!
10. If a = 2, b = 3, c = 4 and d = 5, what is the value of 2a + 3b + c2 - d
[x] 24
[ ] 26
[ ] 28
[ ] 30
4 + 9 + 16 - 5 = 24. Don't forget 'IF IN DOUBT, WRITE IT DOWN AND WORK IT OUT!
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