Video: KS2-M16 • Paper 3 • Question 2

This table shows the temperature at 9 am on three days in January. What is the difference between the temperature on January 1 and the temperature on January 8? On January 22, the temperature was 7 degrees lower than on January 15. What was the temperature on January 22?

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Video Transcript

This table shows the temperature at nine am on three days in January. We can see that on January the first, the temperature was plus five degrees C. On January the eighth, minus four degrees C. And on January the 15th, plus one degrees C. What is the difference between the temperature on January the first and the temperature on January the eighth? And then there’s a second part to the question. On January the 22nd, the temperature was seven degrees lower than on January the 15th. What was the temperature on January the 22nd?

Before we answer both parts to the problem, let’s have another look at the table. We can see that it shows the temperature on three different days: the first, the eighth, and the 15th of January. What do we notice about those temperatures before we start?

We can see that the first and the last temperatures are positive numbers. They have a plus sign in front of them. Plus five degrees and plus one degrees. But we need to be very careful with the temperature on January the eighth. Look how it’s a negative number. This is a temperature below zero. And it’ll be very easy to read this quickly and to think that we’re talking about four degrees, while we’re not. It’s minus four degrees. This is important and we’ll come to this in a second.

Let’s look at the first part of the problem. What is the difference between the temperature on January the first and the temperature on January the eighth? The temperature on January the first is plus five degrees or five degrees. And as we’ve seen already, the temperature on January the eighth is a negative number, minus four degrees. So the question is asking us to find the difference between five and minus four.

An easy mistake to make would be to say that the difference between four and five is one. But remember, we’re not looking for the difference between four and five. We’re looking for the difference between minus four and five. To count from five to minus four, we need to count back to zero and then count below zero. To get from five to zero, we take away five. And then to get from zero to minus four, we take away another four. We’ve made a jump of five degrees and a jump of four degrees. Five plus four equals nine degrees in total. And so the difference in temperature between the two days is nine degrees.

Let’s look at the second part of our problem. On January the 22nd, the temperature was seven degrees lower than on January the 15th. What was the temperature on January the 22nd? So the question is asking us to find the temperature for January the 22nd. The temperature on January the 15th was plus one degrees C. And we’re told that the temperature a week later was seven degrees lower. So we can use this fact to help us find out the missing value.

What is seven degrees lower than one? How can we count backwards seven from one? Let’s sketch another number line to help us understand. Start on one degrees or plus one. If we can’t back one or take away one degree, we get to zero. We still need to count back another six. What is six less than zero? Six degrees less than zero is minus six. Seven degrees lower than one is minus six.

Let’s make sure we don’t forget that minus sign in our answer. So the difference between the temperature on January the first and January the eighth was nine degrees. And the temperature on January the 22nd was minus six degrees. We knew we had to be very careful about how we treated positive and negative numbers. So we found both answers by using number lines to help.

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