Why does my child think 12 km in 30 minutes is a speed of 0.4 km/h?
Your child leaves the units as they are, such as 12 km in 30 minutes as 12 ÷ 30 km/h instead of 24 km/h.
Why children do it
The numbers are right there, so it's tempting to divide them as they are. But a speed in km/h needs hours: 30 minutes is half an hour, so 12 km in half an hour is 24 km/h. Saying the units aloud at each step catches it.
How to help at home
Ask them to say the units out loud at each step: minutes, or hours? centimetres, or metres?
The right way
A cyclist rides 12 km in 30 minutes. What is the average speed in km/h?
- 12 ÷ 0.530 minutes is half an hour, so divide by 0.5 hours.
- 2424 km for each hour.
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