How to see it
Learn a few anchors. One inch is about 2.5 centimetres. One pound is about 450 grams. Five miles is about 8 kilometres. One gallon is about 4.5 litres. A foot is 12 inches. A stone is 14 pounds. Before you calculate, decide which unit is bigger. An inch is bigger than a centimetre, so there are fewer inches than centimetres.
A worked example
Twenty miles. Five miles is 8 kilometres, and 20 is four lots of 5, so 4 times 8 is 32 kilometres. For the rope: 60 divided by 2.5. 24 inches.
The common mistake: converting in the wrong direction
The trap is multiplying when you should divide. 60 times 2.5 gives 150 inches, a rope longer than a room. Inches are bigger, so the number must shrink.
Try one
About how many centimetres is 10 inches?
- A4
- B25
- C250
- D10
Show the answerHide the answer
Right: B, 25