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