# Maths: The three trickier types of questions

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## Finding the percentage change

The formula: percentage change = change / original X 100

Put into practise: A trader buys watches for £5 and sells them for £7. Find his profit as a percentage.

percentage profit = 2/5 X 100 = 40%

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## Finding the original value

1) Write the amount in the question as a percentage of the original value.
2) Divide to find 1% of the original value.
3) Multiply by 100 to give the original value. (=100%).

Put into practice: A house increases in value by 20% to £72,000 find what it was worth before the rise.
1) An increase of 20% means £72,000 represents 120% of the original value .
2) Divide by 120 to find 1% of the original value
3) Then multiply by 100.

£72,000 =120%
/120
£600= 1%
X100 £60,000 = 100%
So the original value was £60,000

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## Simple interest

Simple interest means that a certain percentage of the original amount only is paid at a regular intervals (usually once a year). So the amount of interest is the same very time it's paid.

Put into practise: Regina invests £380 in an account which pays 3% simple interest per annum. How much interest will she earn in 4 years?

1) Work out the amount of interest earned in one year

380/100 X 3= £11.40

2) Multiply by 4 to get the total interest for 4 years.

4 X £11.40 = £45.60

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