Use a symbol
Instead of writing a box, we can write n.
The letter stands for the unknown value.
Year 6
Express missing number problems algebraically
Use a letter to represent an unknown number in a calculation or problem.
Lesson overview
Algebra
A missing number can be shown with a letter symbol. This helps us write the structure of the problem clearly before solving it.
Match words to algebra
A number plus 7 can be written as x + 7.
a number plus 7
x + 7
The letter can be any symbol chosen for the unknown.
Worked example
Write this algebraically: a number multiplied by 3 is 18.
- Let the unknown number be a.
- Multiplied by 3 becomes 3a.
- The result is 18.
- So the algebraic statement is 3a = 18.
Try it
Think about the unknown number first, then write the rest of the structure around it.
Question 1
A number plus 5 is 12 can be written as:
a number plus 5 is 12
n
Question 2
A number multiplied by 3 is 18 can be written as:
Question 3
Which expression means a number take away 7?
x
take away 7
Question 4
Which equation matches: double a number is 20?
Question 5
Which letter can stand for an unknown?
Common mistakes
Watch for these when working through the lesson.
- Using the letter as a label instead of a number.
- Writing the operations in the wrong order from the words.
- Forgetting the equals sign in an equation.
Related topics
These ideas fit closely with this lesson.
- Use simple formulae.
- Find pairs of numbers that satisfy an equation with two unknowns.