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How Many Containers in One Cup / Cups in One Container?

Alignments to Content Standards: 6.NS.A.1

1. If $\frac12$ cup of water fills $\frac23$ of a plastic container, how many containers will 1 cup fill?

• Solve the problem by drawing a picture.

• Which of the following multiplication or divisions problems represents this situation? Explain your reasoning.

$$\frac12 \times \frac23=? \qquad \frac12 \div \frac23=? \qquad \frac23 \div \frac12=?$$
• Solve the arithmetic problem you chose in part (3) and verify that you get the same answer as you did with your picture.

2. If $\frac12$ cup of water fills $\frac23$ of a plastic container, how many cups of water will the full container hold?

• Solve the problem by drawing a picture.

• Which of the following multiplication or divisions equations represents this situation? Explain your reasoning.

$$\frac12 \times \frac23=? \qquad \frac12 \div \frac23=? \qquad \frac23 \div \frac12=?$$
• Solve the arithmetic problem you chose in part (3) and verify that you get the same answer as you did with your picture.

IM Commentary

These two fraction division tasks use the same context and ask “How much in one group?” but require students to divide the fractions in the opposite order. Students struggle to understand which order one should divide in a fraction division context, and these two tasks give them an opportunity to think carefully about the meaning of fraction division.

The purpose of this problem is to help students deepen their understanding of the meaning of fractions and fraction division and to see that they get the same answer using standard algorithm as they do just reasoning through the problem. Later they can build on this to explain why dividing by, for example, $\frac23$ is the same as multiplying by $\frac32$ based on the meaning of multiplication and division by fractions.

Solution

• Below is a series of pictures that can be used to solve the problem as well as an explanation of what the picture represents at each step.

$\frac12$cup of water fills $\frac23$ of a plastic container, as shown below.

We know that $\frac12$ cup of water fills $\frac23$ of a plastic container. The question is asking, "How many containers can be filled with 1 cup of water." Since 1 cup of water is twice as much as $\frac12$ cup of water, we know that 1 cup of water will fill twice as much of a container.

The answer to the question can easily be found by relabeling the above picture.

Twice as much water will fill twice as many containers. Thus, 1 cup of water will fill $\frac43$ of a container.

• We are looking for the number of containers filled by 1 cup, and we know $\frac12$ of that unknown amount fills $\frac23$ of a container. We can write this symbolically as $$\frac12 \times ? = \frac23$$ which is equivalent to $\frac23 \div \frac12$ Computing this by multiplying by the reciprocal, we find that \begin{align} \frac23 \div \frac12 &=\\ &=\frac23 \times \frac21 \\ &= \frac43 \end{align}

• When we solved the problem with a picture, we took the fraction of the container filled by $\frac12$ cup and doubled it to find the fraction of a container filled by one cup. This is exactly what we did when we did it through computations: to divide $\frac23$ by $\frac12$ we multiplied it by 2. The two ways of solving it do result in the same answer and even follow the same process.

• Below is a series of pictures that can be used to solve the problem as well as an explanation of what the picture represents at each step.

$\frac12$ cup fills $\frac23$ of the container, as shown in the picture below.

Since the picture represents $\frac23$ of the container, $\frac12$ of $\frac23$ will be $\frac13$ of the container. In the picture below, each piece represents $\frac12$ of $\frac23$, and also represents $\frac12$ of $\frac12$. Since $\frac12 \times \frac12 = \frac14$, each piece also represents $\frac14$ cup.

We have $\frac23$ of the container, but we need to know how many cups it takes to fill $\frac33$ of the container, so if we take 3 of the thirds, we will have the amount of water needed for the full container. This is the same thing as multiplying $3 \times \frac14$.

We can see that $\frac34$ cup is needed to fill the whole container.

• We are looking for the number of cups needed to fill one container, and we know $\frac23$ of that unknown amount requires $\frac12$ a cup. We can write this symbolically as $$\frac23 \times ? = \frac12$$ which is equivalent to $\frac12 \div \frac23$ Computing this by multiplying by the reciprocal, we find that \begin{align} \frac12 \div \frac23 &=\\ &=\frac12 \times \frac32 \\ &= \frac34 \end{align}

• When we solved the problem using a picture, we found $\frac12$ of the amount (because we knew it was 2 thirds of the container, and so $\frac12$ of that would be 1 third of the container) and then we multiplied that amount by 3 to get the amount in the whole container. This is exactly what we did when we multiplied by the reciprocal of $\frac23$: multiplying by $\frac12$ and then by $3$ is the same as multiplying by $\frac32$. So whether we solve it with a picture or via computation, we get the same answer (in fact, the process itself is the same).

over 2 years

removed

Cam says:

over 2 years

Indeed! Thanks for the catch.

Peggy says:

over 2 years

I'm confused by something in the solution for part b. Under the first picture it says: "Since the picture represents 2/3 of the container, 1/2 of 2/3 will be 1/3 of the container." (so far so good) "In the picture below each piece represents 1/2 of 2/3, and also represents 1/2 of 1/4."

This is the part that is wrong to me. It should be representing 1/2 of 1/2. 1/2 of 1/4 =1/8 and we are definitely not talking about eighths here. The text goes on: "Since 1/2 x 1/2 = 1/4 each piece represents 1/4 cup. (this confirms my idea that the 1/2 of 1/4 in the previous sentence is a typo)

Ann says:

over 3 years

When I read this question, I feel it most closely aligns with 6.RP.3 and 7.RP.1. The order of division may not be as confusing for students if they set up a ratio table, or double number line diagram to model the given ratio relationship. Although complex fractions are not formally used to find a unit rate until Grade 7, that is what is going on here. Looking at the complex fraction 1/2/2/3, we can use equivalent fractions and scale up by 2, to say 1/2/2/3 = 1/4/3. For me, it is more natural to "scale up" and think: 1/2 for every 2/3 means 1 for every 4/3. So the answer for Part A is 4/3 or, One and a third containers. For Part B, using the same complex fraction, but making an equivalent fraction with "1" in the denominator (to represent 1 full container), means we need to multiply the denominator by 3/2 to scale up, and therefore, do the same to 1/2, the numerator. Scaling up by 3/2: 1/2/2/3 = 3/4/1. And so the answer to Part B is 3/4 cups.

In the answer key for part a) and b), please note that using a,b, and c's used within each part is confusing, since in the original problem those parts were labeled i), ii) and iii).