Video Transcript
Subtracting Ones from Two-Digit
Numbers with Regrouping
In this video, we will learn how to
subtract a one-digit number from a two-digit number when crossing a 10 and model
this with place value equipment. The number 52 is a two-digit
number. Its tens digit is a five, and we’ve
modeled this using five 10s blocks. 10, 20, 30, 40, 50. Five 10s are worth 50. The two digit in the number 52 is
worth two ones, and we’ve modeled this with two ones blocks. We’re subtracting a one-digit
number from our two-digit number. The number seven only has one
digit. It’s worth seven ones.
In this video, we’re learning how
to use column subtraction to help us subtract a one-digit number from a two-digit
number. When we add or subtract using the
column method, we always start calculating in the ones column first. 52 has two ones, and we have to
subtract seven. We have a problem. Two is less than seven. We don’t have enough ones to
subtract seven from. We need to regroup. We need to exchange one of our tens
for 10 ones, so let’s take one of our tens and exchange it for 10 ones. Now we have four 10s and 12
ones. So now we have enough ones to
subtract our seven from. We have 12 ones, and we need to
take away seven. 12 subtract seven equals five.
Next, we need to subtract in the
tens column. We had five 10s, and we exchanged
one 10 for 10 ones, so we only have four 10s. And because we’re subtracting a
one-digit number, there’s nothing to subtract from our four 10s. Four 10s take away no tens leaves
us with four 10s. 52 subtract seven is 45. So, we’ve learned that when we’re
subtracting one-digit number from a two-digit number, we always start by subtracting
the ones first. We do this in case we need to
regroup.
The next thing we have to do is
regroup. We didn’t have enough ones to
subtract seven from 52 because 52 only has two ones. So we have to take one of our tens
and exchange it for 10 ones. Now we have 12 ones and four
10s. Once we’ve regrouped, we have
enough ones to subtract from. Then we can subtract the tens. Because we’re subtracting a
one-digit number from a two-digit number, the tens digit has nothing to subtract
from. So we just need to write the number
of tens in the tens place. 52 subtract seven is 45. Let’s try and put into practice
what we’ve learned now by answering some questions.
Mia is calculating 33 subtract
four. She showed 33 using place value
blocks. After she regroups and subtracts
four, how many tens and ones will be left?
In this question, we have to help
Mia calculate 33 subtract four. The number 33 is a two-digit
number. And Mia’s modeled this using place
value blocks. 33 has three 10s, worth 30, and
three ones, which are worth three.
The second part of the question
tells us that Mia has to regroup to subtract four. We know that four is more than
three. There are not enough ones in the
number 33 to subtract four from. So we need to take one of our tens
and exchange it for 10 ones. Now we have two 10s and 13
ones. Now we have enough ones to subtract
our four from. 13 take away four is nine. We’ve got nine ones left. We had three 10s. We exchanged one 10, so we only
have two 10s left. And there’s nothing to subtract
from these two 10s in the tens place. So 33 subtract four is 29. After Mia regroups and subtracts
four, she will have two 10s and nine ones left.
Subtract eight from 66.
In this question, we’re subtracting
a one-digit number from a two-digit number. 66 has two digits, and eight has
one. When we’re subtracting from a
two-digit number, we always start by subtracting the ones. But six is less than eight. We don’t have enough ones to
subtract from. So we need to regroup. We need to take one of our six 10s,
which would leave us with five 10s, and exchange it for 10 ones. So instead of having six ones, we
have 16 ones. Now we have enough ones we can
subtract in the ones column. What is 16 take away eight? It’s eight.
And because we’re subtracting a
one-digit number from a two-digit number, there’s nothing to subtract from our
tens. We had six 10s. We exchanged one of our tens for 10
ones, leaving us with five 10s. And five 10s take away no tens
leaves us with five 10s. 66 take away eight is 58. First, we regrouped one of our tens
and exchanged it for 10 ones. Next, we subtracted the ones. 16 take away eight is eight. Then we subtracted the tens. Five 10s take away no tens is five
10s. We subtracted eight from 66,
leaving us with 58.
A car park has 41 spaces. Two spaces are taken. How many are left?
In this question, the car park has
41 spaces. If two spaces are taken, to
calculate how many spaces are left, we need to subtract the two spaces that are
taken from the 41 spaces in the car park. So we’re subtracting a one-digit
number from a two-digit number. We can model the number 41 using
four 10s blocks and a ones block. When we’re subtracting using the
column method, we always start with the ones first. We’ve got one in the ones place,
and we need to subtract two. We don’t have enough ones to
subtract from. So we need to take one of our tens,
leaving us with three 10s, and exchange it for 10 ones. Now we have 11 ones.
Now that we have enough ones, let’s
subtract in the ones place. 11 ones take away two ones leaves
us with nine ones. Now what we need to do is subtract
in the tens column. Because two is a one-digit number,
there’s nothing to subtract. We had four 10s. We exchanged one of our tens for 10
ones, which leaves us with three 10s. 41 subtract two equals 39.
What have we learned in this
video? We have learned how to subtract a
one-digit number from a two-digit number with regrouping.
