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Subtracting Ones from Three-Digit Numbers
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In this video, we will learn how to use models to subtract a one-digit number from a three-digit number and investigate which digits in the number change.
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In this video, we’re going to be subtracting one-digit numbers from three-digit numbers.
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We know that three-digit numbers have a hundreds digit, a tens digit, and a ones digit.
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The hundreds digit in the number 279 is worth two 100s.
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The tens digit is worth seven 10s.
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And we know that seven 10s are 70.
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The nine digit in the number 279 is worth nine ones.
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If we were to subtract six from this number, we would be left with three ones.
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We know that six is a one-digit number.
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So to subtract six from 279, all we needed to do was take away six of the ones.
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In this video, we’re going to subtract one-digit numbers from three-digit numbers.
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And we’re going to look closely at how the digits change.
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Another way we could subtract ones from a three-digit number is to use a number line.
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If we’re subtracting ones, we just need to count back in ones.
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So if we wanted to subtract five from 799, we would need to start at number 799 and count back five.
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798, 797, 796, 795, 794.
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We started at number 799 and counted back in ones five times.
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The number we reached is 794.
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So far, we’ve learned that if we subtract a one-digit number from a three-digit number, we’re subtracting ones.
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And we can do this using a place value chart and subtract from the ones digit.
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We could also model the number using 100s, 10s, and ones counters or place value blocks.
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Or we could count back in ones along a number line.
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Let’s have a go at some questions now where we need to subtract a one-digit number from a three-digit number.
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Which number is five less than 508?
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In this question, we have to subtract.
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To find the number five less than 508, we need to calculate 508 subtract five.
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If we write the number 508 in a place value chart, that will help us think about the value of the digits.
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Our three-digit number is 508.
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And we’re subtracting five, which is a one-digit number.
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508 has eight ones.
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We just need to subtract five.
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What is eight take away five?
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It’s three.
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If we have eight ones and subtract five, there will be three ones left.
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508 take away five equals 503.
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The number which is five less than 508 is 503.
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Count in ones to subtract.
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358 subtract six equals what?
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We have to subtract six from 358.
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And we’re given a number line which starts at 350 and ends at 360.
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The question tells us to count in ones to subtract.
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When we’re subtracting, we need to count back along the number line.
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We need to start at 358 and count back six.
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And if we count in ones, we need to count back six times.
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One, two, three, four, five, six.
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We started at 358 on the number line.
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We counted back in ones six times.
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That took us to the number 352.
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358 subtract six equals 352.
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To subtract the six ones from our three-digit number, we counted back in ones.
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We counted back six times until we reached the number 352.
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Find the difference by first subtracting ones.
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246 subtract five equals what?
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When we see the phrase “find the difference,” that tells us this is a subtraction problem.
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We have to subtract five from 246.
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The question tells us which method to use to subtract.
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First, we have to subtract the ones.
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And we’ve been given this part–whole model or cherry model to help us partition the number.
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The number 246 is a three-digit number, so it has a hundreds, tens, and ones digit.
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The six digit in the number 246 is worth six ones.
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So we can write the number of ones in this part of our model.
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How much is the rest of the number worth?
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We know the two digit is worth 200 and four 10s are worth 40.
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So we could partition the number 246 into 240 and six.
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We’ve broken the number apart into an amount of ones and an amount of hundreds and tens.
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Here are six counters to represent our six ones.
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We have to subtract five because we’re calculating 246 subtract five.
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One, two, three, four, five.
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If we have six ones and subtract five, we’re left with one.
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246 subtract five equals 241.
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We found the difference by first subtracting the ones.
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Elizabeth buys a notebook that has 109 pages.
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She completes her assignments in seven pages.
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How many blank pages are left in the notebook?
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This is a subtraction problem.
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Elizabeth has 109 pages in her notebook to begin with.
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And she uses seven pages to complete her assignments.
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To find out how many pages she has left in her notebook, we need to subtract seven from 109.
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Did you notice that we’re subtracting a one-digit number from a three-digit number?
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We can start by first subtracting the ones.
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Seven only has a ones digit, so we just need to subtract seven ones from our nine ones.
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We need to start at the number 109 and count back in ones seven times.
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One, two, three, four, five, six, seven.
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109, 108, 107, 106, 105, 104, 103, 102.
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If Elizabeth’s notebook has 109 pages and she uses seven pages, there will be 102 pages left in her notebook.
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109 subtract seven equals 102.
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What have we learned in this video?
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We have learned how to subtract a one-digit number from a three-digit number by first subtracting ones.
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We also learned how to subtract ones from three-digit numbers using place value charts and counters or by counting back in ones.