2 · Substitution
Find the value of an expression by replacing letters with numbers.
What your child learns
- Replace each letter with its value
- Follow the order of operations
- Use brackets for negative numbers
Example
If x = 4, then 3x + 2 = 3 × 4 + 2 = 14.
Mini lesson: Substitution
Goal: Find the value of an expression when you know what the letter is worth.
To substitute, replace each letter with its number, then work it out using the order of operations.
- Put brackets around negative numbers: if x = −2, then 3x = 3 × (−2) = −6.
- x² means x × x, not 2 × x.
Worked example
Find 4x + 3 when x = 5.
- Replace x with 5: 4 × 5 + 3
- Multiply first: 20 + 3
- Answer: 23
Common mistakes
- Writing 4x as 45 when x = 5 (it is 4 × 5).
- Squaring −3 and getting −9 (it is +9).
Practice questions
A taste of the questions in BloomTracks – the app makes fresh ones every time and adjusts the level as your child answers.
-
Question 1 · Level 1
If n = 8, what is n + 4?
Show the answer
Answer: 12
8 + 4 = 12.
-
Question 2 · Level 3
If x = 1 and y = 8, find 5x + 6y.
Show the answer
Answer: 53
5 + 48 = 53.
-
Question 3 · Level 5
Find 2x² − 2y when x = −1 and y = −5.
Show the answer
Answer: 12
2 − (−10) = 12.
Try it at home
Make a 'secret number' game: x is 5, what is 2x + 1?
Key takeaway
x² means x × x, not 2 × x.
Practise 2 · substitution with Bloomie
Bloomie reads every question aloud, listens to spoken answers and adapts the difficulty one question at a time.