How much more, and how many times
A photo is 90 mm (millimetres) wide and 60 mm tall. You shrink it two ways. Which way keeps its shape?
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Step 1 of 3
- Two ribbons: 18 m and 12 m.
- Subtract: 18 − 12 = 6 m more.
- Divide instead: 18 ÷ 12 = 1.5.
The rule
Cut both sides to a third and the photo keeps its shape. It is then 30 mm by 20 mm. Taking 20 mm off both sides changes the shape.
You can compare two amounts in two ways. How much more? Subtract. How many times? Divide.
Take two ribbons, 18 m and 12 m. Then 18 − 12 = 6 m more. And 18 ÷ 12 = 1.5 times. Comparing by dividing gives a ratio, written 18 : 12.
Both sides must be multiplied by one number to keep the shape. Cutting both sides to a third multiplies each by 1/3. Taking 20 mm off both sides is not multiplying.