KS3

How to find the original value before % off (decimal method)

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A one-minute video showing how to work out the original value before a percentage change.

Work out original value slideshow

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Click to see a step-by-step guide.
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WHAT YOU NEED: whiteboard, ruler, pens and a calculator.
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STEP 1: Draw a bar: if the whole bar was 100%, estimate where 40% from the right would be and draw a vertical line. Label the left-hand part £12 and shade it in. Now label the whole bar with a question mark. Draw a bar underneath and label it 100%.
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STEP 2: On the bottom bar draw a vertical line directly underneath the vertical line on the top bar. Label it 60%, representing the price after a 40% discount, then shade the area in. Label the right-hand segment 40%, which represents the discount.
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STEP 3: Convert the percentages to decimals by dividing them by 100. So on the bottom bar replace the 60% label with 0.60, the 100% label with 1 and the 40% label with 0.40.
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STEP 4: The bars show that the original amount x 0.60 = £12. This means that £12 ÷ 0.60 = the original amount. Before you use a calculator, can you estimate what the answer might be in your head?
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Use a calculator to check your answer. You’ve done it! Now you know that the original price of the football was £20.

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