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Calculating percentage

In the example: "10% of 50 cars = 5 cars", we have three numbers involved:
  1. The starting number. (50 cars)
  2. The percentage. (10%)
  3. 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?
  1. First, write the percentage as a fraction. Put the percentage on the top and a hundred on the bottom
  2. Then multiply the starting number of cars by the fraction:

Try this

  1. In the applet above, uncheck the right box under the result to hide it.
  2. Drag both orange dots to create a new problem.
  3. Calculate the result
  4. 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?
  1. 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".
  2. Then convert the fraction to a percentage by multiplying by 100:

Try this

  1. In the applet above, uncheck the middle box under the percentage to hide it.
  2. Drag both orange dots to create a new problem.
  3. Calculate the percentage
  4. 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?
  1. 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:
  2. 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

  1. In the applet above, uncheck the left box under the starting number to hide it.
  2. Drag both orange dots to create a new problem.
  3. Calculate the starting number.
  4. Check the box to see how you did.
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