A model that works
A stick is 24 cm long. It burns 0.4 cm every minute. How long is it after 10 minutes?
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The idea
In 10 minutes, 0.4 × 10 = 4 cm burns. The stick is 24 − 4 = 20 cm long.
A simple picture of how something works is a model. Here the model is a straight line.
Length = 24 − 0.4 × minutes.
Here 24 is the starting length in cm. 0.4 cm burns each minute.
A case is one more measurement to check. A model fits when its answer is close to the measurement. Close means within 0.5 cm. Then the line covers that case. A model works only where it fits. Check a new case before you trust it.
Step 1 of 4
- The stick's length drawn as a straight line over time.
- At 25 minutes, the line matches what was measured.
- At 75 minutes, the line goes below zero.
- The stick went out at 60 minutes. Nothing is left to measure.
The line is made for a plain stick. A fan or a thicker stick can burn at another speed. The line may not fit them.