Percentage of an Amount: Mental Methods
Percentage of an amount by mental methods covers fast, non-written strategies for finding percentages mentally, built up from 10%, 5% and 1% of an amount rather than a full written calculation. This speed-focused skill is tested directly in the mental arithmetic and speed sections of GL Assessment and CEM-style 11+ papers, where pupils have only a few seconds to answer each question.
Method
- Start by finding 10% of the amount by dividing by 10, since almost every other percentage can be built from it quickly.
- Halve 10% to get 5%, and halve again to get 2.5% if the question needs it.
- Find 1% by dividing the amount by 100, useful for percentages that are not multiples of 10 or 5.
- Build the target percentage by adding or subtracting combinations of 10%, 5% and 1% blocks, choosing the route with the fewest steps.
- For percentages just over or under a round number, such as 49% or 51%, find the round number first (50%) and adjust by adding or subtracting 1%.
- Practise the common percentages 50% (half), 25% (a quarter) and 75% (three quarters) so you can recall them instantly without building up.
Worked example
Find 65% of 240 mentally, showing the building blocks you used.
- Find 50% of 240 by halving: 120.
- Find 10% of 240 by dividing by 10: 24.
- Find 5% of 240 by halving the 10% value: 12.
- Combine 50% + 10% + 5% to make 65%: 120 + 24 + 12.
- Final answer: 120 + 24 + 12 = 156.
Practice questions
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Q1Find 10% of 350.Show answer
Answer: 35
Q2Find 5% of 80.Show answer
Answer: 4
Q3Find 25% of 96.Show answer
Answer: 24
Q4Find 40% of 150 using 10% as a building block.Show answer
Answer: 60 (10% = 15, so 40% = 15 x 4 = 60)
Q5Find 1% of 600.Show answer
Answer: 6
Q6Find 99% of 400 using a quick mental shortcut.Show answer
Answer: 396 (400 minus 1%, which is 4)
Q7Explain why finding 10% of an amount is a useful first step for finding many other percentages mentally.Show answer
Answer: 10% is found simply by dividing by 10, and many other percentages, such as 5%, 20%, 30% and 15%, can then be built quickly by halving, doubling or adding multiples of that 10% value, without needing a full written calculation.
Exam-style questions
Written in the style of a 11+ exam paper, with a full mark scheme.
Find 45% of 80 mentally, showing the building blocks you used.
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A market stall sells 220 items in a day. 15% of the items are apples, 20% are pears, and the rest are oranges. Using mental methods, find the number of oranges sold.
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Free printable worksheet
Want more practice on paper? Download the percentage of an amount: mental methods worksheet pack - 6 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 45 of 11+ Maths Workbook 1, the whole course as one free printable PDF.
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