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Shape and Space
Statistics and Probability
If you have to turn a percentage into a decimal, just divide by 100. For example, 25% = 25/100 = 0.25.
Find 25% of 10 (remember 'of' means 'times').
% change = new value - original value × 100
The price of some apples is increased from 48p to 67p. By how much percent has
the price increased by?
% error = error × 100
Nicola measures the length of her textbook as 20cm. If the length is actually
17.6cm, what is the percentage error in Nicola's calculation?
Original value = New value × 100
Amish buys a stamp collection and makes a 35% profit by selling it for £2700.
Find the cost of the collection.
Percentage Increases and Interest
New value = 100 + percentage increase × original value
£500 is put in a bank where there is 6% per annum interest. Work out the amount
in the bank after 1 year.
If in this example, the money was left in the bank for another year, the £530
would increase by 6%. The interest, therefore, will be higher than the previous
year (6% of £530 is more than 6% of £500). Every year, if the money is left
sitting in the bank account, the amount of interest paid would increase each
year. This phenomenon is known as compound interest.
(100 + %change)no of years × original value
So if the £500 had been left in the bank for 9 years, the amount would have increased to:
500 × (1.06)9 = £845
New value = 100 - percentage decrease × original value
At the end of 2003 there were 5000 members of a certain rare breed of animal
remaining in the world. It is predicted that their number will decrease by 12%
each year. How many will be left at the end of 2005?
The compound interest formula above can also be used for percentage decreases. So after 4 years, the number of animals left would be:
5000 x [(100-12)/100]4 = 2998