Video: Addition and Subtraction of Decimal Numbers

Daisy goes jogging every day. If she jogged for 7 hours this week, complete the given table.

03:45

Video Transcript

Daisy goes jogging every day. If she jogged for seven hours this week, complete the given table.

We can see that the first row of our table shows the days of the week. So there are seven separate columns. And for each day of the week, in our second row of the table, it shows the number of hours that Daisy spends jogging each day. Well, all apart from one day, we can see that Saturday’s time is missing. The question tells us that Daisy jogged for seven hours this week. So we need to use this fact to help us work out how many hours that she jogged on Saturday. We know she definitely did jog on Saturday because she goes jogging every day.

To start with, let’s calculate how many hours we can see in the table at the moment. On Monday, Daisy jogged one hour. On Tuesday, she jogged another hour. That’s a total, now, of two hours. Let’s make a note of that. She jogged another hour on Wednesday. So that’s three hours now in total. Another hour on Thursday. On Friday, she jogs 0.5 hours. That’s the same as half an hour. And if we add 0.5 to four, we get a total of 4.5 or four and a half hours. We don’t know what Saturday’s time is. But we do know that Daisy jogged one more hour on Sunday.

So at the moment, our table shows that she jogged 5.5 or five and a half hours. But the question tells us that Daisy jogged for seven hours this week. To find out how many hours Daisy jogged on Saturday, we need to subtract 5.5 from seven. Now, there are several ways we could find the answer to this. For example, we could start with 5.5 hours and count on until we get to seven. We could use a number line to help us. But in this video, we’re going to use the column method. Let’s subtract 5.5 from seven.

Now, if we look carefully at our subtraction, we don’t have anything in the tenths place in the first number. It’s just the number seven. But this is going to make it difficult for us. We need to subtract from something in the top number. So we need to write a decimal point and use zero as a placeholder. The number is still seven, but now we can subtract five-tenths. And the way we’re going to subtract five-tenths is by exchanging. We can’t take away five-tenths from zero. So we can take one one — instead of seven ones, we now have six ones — and exchange it for ten-tenths. Because we know ten-tenths are the same as one one. Ten-tenths take away five-tenths leaves us with five-tenths.

Now, as we move along from right to left, let’s make sure we put the decimal point so it’s aligned with the other decimal points. Now, we just need to subtract the ones. Six take away five equals one. The remaining amount to take us from 5.5 up to seven is one and a half hours or 1.5. And so, if Daisy jogged for seven hours this week, we need to complete the table using the number 1.5.

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