Video: Subtracting by Decomposing to Make 10

Sophia is subtracting by making 10. Which calculation should she do first? Which calculation should she do next?

02:47

Video Transcript

Sophia is subtracting by making 10. 13 subtract five equals what? Which calculation should she do first? 13 take away two equals 11 or 13 take away three equals 10. And then, which calculation should she do next? 10 take away two equals eight or 10 plus two equals eight.

And maybe you’ve noticed one of these calculations doesn’t make sense, but let’s go through the question from the start and find out what Sophia is doing here. We’re told that she’s subtracting by making 10. And we can see from the picture the two numbers that Sophia’s subtracting. She’s trying to find the answer to 13 take away five.

Now, she could find the answer just by starting with the number 13 and counting back five. But we’re told that she wants to find the answer by making 10. First, she can take away part of the number five to get to 10. And then, she can take away the rest of the number five from 10 because subtracting to and from 10 is quite quick to do.

Here are 13 counters. And we can see from the diagram that Sophia has split up the five that she wants to take away into two jumps. First, she takes away three. As we’ve said already, we know why she does this; she wants to make 10. She knows that 13 take away three equals 10. So, we know the calculation that she needs to do first is 13 subtract three equals 10.

Now, Sofia knows that three and two go together to make five. She’s already taken away three out of five. Now, she needs to just subtract the remaining two. And because she’s made 10, she takes away two from 10, which is very quick to do. Now, if we look carefully to our possible answers, only one of them makes sense. 10 plus two doesn’t make eight.

But let’s think about how Sophia knows that 10 take away two equals eight. Well, she knows that two and eight go together to make 10. She can subtract from 10 really quickly. And so, the second calculation she needs to do is 10 subtract two equals eight.

Sophia found the answer to 13 subtract five by splitting up five into three and then two. The calculation she should do first is 13 take away three equals 10. And her next calculation should be 10 take away two equals eight.

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