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Snow Day


Alignments to Content Standards: 3.NF.A.3

Task

Alec and Felix are brothers who go to different schools. The school day is just as long at Felix' school as at Alec's school. At Felix' school, there are 6 class periods of the same length each day. Alec's day is broken into 3 class periods of equal length. One day, it snowed a lot so both of their schools started late. Felix only had four classes and Alec only had two. Alec claims his school day was shorter than Felix' was because he had only two classes on that day. Is he right?

IM Commentary

The purpose of this task is for students to investigate a claim about a comparison of two fractions in a context. Many fraction problems are set in food contexts or a situation where a physical thing is being shared. It is important for students to work on more abstract quantities like time as well. 

The Standards for Mathematical Practice focus on the nature of the learning experiences by attending to the thinking processes and habits of mind that students need to develop in order to attain a deep and flexible understanding of mathematics. Certain tasks lend themselves to the demonstration of specific practices by students. The practices that are observable during exploration of a task depend on how instruction unfolds in the classroom. While it is possible that tasks may be connected to several practices, only one practice connection will be discussed in depth. Possible secondary practice connections may be discussed but not in the same degree of detail. 

This particular task helps to illustrate Mathematical Practice Standard 3, Construct viable arguments and critique the reasoning of others. Students are asked to critique the reasoning of Alec’s claim that his school day was shorter than his brother’s. This type of task provides students with an opportunity to distinguish a reasonable explanation from that which is flawed.  If there is a flaw in the argument they can further explain why it is flawed. Students will need to determine what information is necessary to calculate in order to support or dispute the claim. To distinguish if Alec’s claim is true or false, students will have to uncover the fractional part of the day Alec and Felix were at school and not just rely on the number of classes they each attended. Learning how to argue whether a claim is true or false concisely and precisely becomes a routine part of a student’s mathematical work. 

Solution

Felix has six equal class periods each day.

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So each class period lasts for $\frac16$ of the day.

Alec has three equal class periods each day.

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So each class period lasts for $\frac13$ of the day.

Felix only had 4 class periods, so he went to school for $\frac46$ of a full day. Alec only had 2 class periods, so he went to school for $\frac23$ of a full day.

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But a full day is equal for the two brothers, so two of Felix' class periods are the same length as one of Alec's. The brothers actually went to school for the same amount of time on the snow day.