
Calculating percentage
In the example:
"10% of 50 cars = 5 cars", we have three numbers involved:
 The starting number. (50 cars)
 The percentage. (10%)
 The result. (5 cars)
We can find any one of them if we know the other two.
The three cases are described below. They each use the applet below to illustrate the techniques.
1. Calculating the result
Given the percentage and starting number (10% of 50 cars). How many cars is that?
 First, write the percentage as a fraction. Put the percentage on the top and a hundred on the bottom
 Then multiply the starting number of cars by the fraction:
Try this
 In the applet above, uncheck the right box under the result to hide it.
 Drag both orange dots to create a new problem.
 Calculate the result
 Check the box to see how you did.
2. Calculating the percentage
Given the starting number (50 cars) and the result (5 cars). What percentage is that?
 First, write the relationship as a fraction. Put the result (5) on the top and the starting number on the bottom:
This would be read as "5 out of 50 cars".
 Then convert the fraction to a percentage by multiplying by 100:
Try this
 In the applet above, uncheck the middle box under the percentage to hide it.
 Drag both orange dots to create a new problem.
 Calculate the percentage
 Check the box to see how you did.
3. Calculating the starting number
Given the result and the percentage, find the original starting number.
For example if I have 5 cars and I know that is 10% of the total, how many cars are there in total?
 We know that 10% of the total is 5, so first find what 1% would have been. It would be a tenth of the 5 cars so
write a fraction with the result on the top and the percentage on the bottom:
 If that is 1% of the total, then the total must be 100 times that, so multiply by 100 to get the total number of cars:
Try this
 In the applet above, uncheck the left box under the starting number to hide it.
 Drag both orange dots to create a new problem.
 Calculate the starting number.
 Check the box to see how you did.
Other fractions, percents, ratios topics
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