We can calculate using unknown numbers by using letters in the place of the unknown numbers.

When we use letters instead of unknown numbers the answer will be an expression.

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If we multiply n by 5 we get n × 5
We don't write a times sign in algebra, so we simplify n × 5 to 5n
(We write the number first to not mistake 5 × n for n5)

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If we have 5 more than n we have n + 5

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If we have 5 less than n we have n - 5

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If we have n divided by 5 we have n ÷ 5 or n⁄5

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Example 1: Apples cost 30p each. Write an algebraic expression for the cost of x apples.

We need to multiply 30 by x.

30 × x = 30x

The total cost is 30x pence.

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Example 2: William has m marbles. Simon has 10 more marbles than William. Write an algebraic expression for the numbers of marbles Simon has.

We need to add 10 onto m

The answer is m + 10

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Example 3: You buy 5 cups of tea for £p
Write an expression for the price of one cup of tea.

We need divide p by 5

The answer is p⁄5
£ p⁄5

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Try these: