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In this activity, we will learn how to increase and decrease numbers by a given percentage using a calculator.
Methods and Examples
A whole number or quantity is 100%.
Increasing a number by 10% gives 100% + 10% = 110% of the number or 1.1 × the number.
So 25 cm increased by 10% is 1.1 × 25 = 27.5 cm.
Decreasing a number by 15% gives 100% - 15% = 85% of the number or 0.85 × the number.
So 25 cm decreased by 15% is 0.85 × 25 = 21.25 cm.
Let's try one more example of each, shall we?
What is 73% increased by 25%? 25% extra means 100% plus 25% which makes 125%. So we need to multiply by 1.25, we say that our multiplier is 1.25.
73 x 1.25 = 91.25
What if 73% decreased by 16%?
100 - 16 = 84, so our multiplier becomes 0.84
73 x 0.84 = 61.32
Does this make sense? Let's have a go at some questions then and if you need to check back to these instructions, just click on the red button at any point.
Question
/ 10Mr Shepherd
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Teacher explanation