Video: KS1-M18 • Paper 1 • Question 17

54 − 20 = _.


Video Transcript

54 take away 20 equals what.

In this problem, we need to start with 54 and then take away 20. In other words, what’s 20 less than 54. This is the sort of calculation where we can use what we know about place value to help. 54 is made up of five tens and four ones. The number that we need to take away 20 just contains two tens. Let’s take away one ten at a time until we’ve taken away two tens. Here’s 54. And this is what happens if we take away one lot of ten. We’re left with four tens and four ones, which is the same as 44. Let’s take away another lot of ten. Here’s 44. And here’s what happens if we take away another ten. We’re left with three tens and four ones. 44 take away 10 equals 34.

By taking away 10 and then another 10, this is the same as taking away two lots of ten or 20. 54 take away 20 equals 34. Notice how only the digit in the tens place changes as we take away the tens. The ones digit stays the same, 54, 44, 34. The number of tens in our number change from five tens to three tens because five take away two equals three.

Another way we could find the answer is using a hundred square to help us. Here’s part of a hundred square. The good thing about hundred squares is that we can count in tens quickly using them. If we look at the numbers that go down a column, it’s the same as counting forwards in tens. And if we look at the numbers going up a column, this is the same as counting backwards in tens, which is what we need to do when we take away 20. Start with 54 and then move up two squares to count back 20, 54, 44, 34. If we count back two tens from 54, we reach 34.

And so we can say 54 take away 20 equals 34.

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