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Running a Mile


Alignments to Content Standards: 5.NF.B.5

Task

Curt and Ian both ran a mile. Curt's time was $ \frac89$ Ian's time. Who ran faster? Explain and draw a picture.

IM Commentary

There is a subtlety worth noting: we are given information about the boys' times but asked about their speeds. Since the distance they run is the same, this isn't difficult to reason through, but teachers need to be aware of this.

The two solutions reflect different competencies described in 5.NF.5. The first solution uses the idea that multiplying by a fraction less than 1 results in a smaller value. The second actually uses the meaning of multiplying by $\frac89$ to explain why multiplying by that fraction will result in a smaller value.

Solutions

Solution: Scaling by a number less than 1

To find Curt's time, you would multiply Ian's time by $\frac89$. Since we are multiplying Ian's time by a number less than 1, Curt's time will be less than Ian's time. The picture shows Ian's time multiplied by 1 above the number line and Ian's time multiplied by $\frac89$ below the number line.

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Since they both ran the same distance but Curt ran it in less time, he must have been running faster.

Solution: Using the meaning of fraction multiplication

Curt's time is $\frac89 \times$ Ian's time. That means that if you divide Ian's time into 9 equal time intervals and take 8 of those intervals, you will have Curt's time. So Curt's time to run a mile is less than Ian's and he must be going faster.

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EllenM says:

8 months

Another possible visual could be two lines plotted on a coordinate grid with time going across the x-axis and distance on the y-axis. Both lines would start at the origin and end at the same height (1 mile) but Curt's line would be steeper. This is not something I would expect a 5th grade student to come up with, but it is something I would be ready for just in case. Personally, I like this visual because it shows the speed as distance over time, or miles per unit time, which can be seen as the slope of the line.

Anthony says:

over 2 years

It is so true that students are taught that when you multiply the number always gets bigger. Again that word 'of' a number is important and can only be understood through a visual. Explain and draw a picture is the key. As a sixth grade teacher students think drawing picture is a primary skill.