Video: Identifying the Appropriate Expression for a Division Problem

There are 81 students in a class. They sit in rows, and each row has 9 students. Choose the expression which tells you how many rows there are. [A] (9 × 81 ) rows [B] (81 − 9) rows [C] (81 ÷ 9) rows [D] (9 + 81) rows

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Video Transcript

There are 81 students in a class. They sit in rows, and each row has nine students. Choose the expression which tells you how many rows there are. Nine multiplied by 81 rows, 81 take away nine rows, 81 divided by nine rows, or nine plus 81 rows.

Here, we’re given four possible ways of working out the number of rows in a class. And if we look at each calculation, the first thing we notice is that they all have a different symbol. The operations are different. So in a way, what this question is asking us is, what do we need to do to find the answer? Do we multiply, subtract, divide, or add. The second thing that we notice about the possible answers is that they all feature the same two numbers. And these are the two numbers that come up in our question.

Firstly, we’re told the total number of students that there are in a class and it’s a big class. There are 81 students. And we’re told that our 81 students are sitting in rows. The next piece of information that we’re given gives us our second number. We’re told that each row has nine students. They’re all equal, and they’re all contain nine students. This means that we can think of the students as making an array, a rectangular shape with rows and columns. We can think of each row as being a group of nine students.

And we know that there are 81 students altogether. So to find the number of rows, we need to find the number of groups of nine there are in 81. This means we need to divide 81 by nine. This is a division problem. And we can see that one of our possible answers is a division calculation, 81 divided by nine rows. So we found the expression that tells us how many rows there are. Let’s quickly just work out the answer. Let’s skip count in nines to find out how many groups of nine there are in 81. Nine, 18, 27. There are four rows of nine in 36, five rows of 45, 54. There are seven groups of nine in 63, eight in 72, and nine groups of nine in 81.

And so if we count the number of rows of students, we can see that there are nine rows altogether. Nine times nine equals 81. And we found the answer by dividing 81 into nine groups. We decided that this was a division question. And the expression that tells us how many rows there are is 81 divided by nine rows.

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