In this video, we’re going to learn
how to add a multiple of 10 to another multiple of 10 and also how to model this
using place-value equipment. Each row in this 100 square is made
up of 10 smaller squares. And so if we want to use this 100
square to count in 10s, we just need to look at the last number in each row. 10, 20, 30, 40, 50, 60, 70, 80, 90,
100. We’ve counted all the way from one
10 through to 10 10s. Did you notice how each of the
numbers that we read has a zero on the end? We call these numbers multiples of
10. And in this video, we’re going to
learn how to add different multiples of 10 together. So we might add 50 and 20, 30 and
40, or maybe have a go at 70 plus 10. All of these additions show a
multiple of 10 being added to another multiple of 10.
Let’s have a think how we can do
this. Let’s try finding the total of
these two multiples of 10. What’s 60 add 30? To help us find the answer, we
could model both numbers using place-value equipment. These are the sorts of maths
equipment that we use to help us split up numbers into 10s and ones. What does the number 60 look
like? We could model the number 60 by
showing six 10s because we know six 10s are the same as 60. 10, 20, 30, 40, 50, 60. Now, let’s model the second number
in our addition. How could we show 30? 30 is the same as three 10s. 10, 20, 30. So to find the total of 60 and 30,
all we have to do is to add together six 10s and three 10s.
Now, is there a number fact that we
could use to help here? We know that six ones or six plus
another three ones equals nine ones. And because we know that six plus
three equals nine, we know the total of six 10s and three 10s will be nine 10s. Nine 10s are the same as 90. 60 plus 30 equals 90.
Let’s try adding another pair of
multiples of 10. What’s the total of 40 and 40? Now, we know that 40 is the same as
four 10s. And because we’re adding 40 and 40
together, we need to find the answer to four 10s add another four 10s. Now, can you spot a simpler
addition fact that we could use to help us here? What about if we use our
place-value equipment to add ones first instead of 10s? Maybe we could use this to
help. Four ones plus another four ones
equals eight ones. And if we know this, then we also
know that four 10s plus another four 10s are going to make eight 10s. And eight 10s is the same as
80. If four plus four equals eight, we
know that 40 plus 40 equals 80.
Let’s try answering some questions
now where we have to add together pairs of multiples of 10. And to help us, we’ll carry on
using place-value equipment.
Add to find the total. Seven plus two equals what. 70 plus 20 equals what.
In this problem, we’re given two
additions to work out. We need to find the total of the
digits seven and two. And then we’re given a pair of
two-digit numbers to add, 70 and 20. These two numbers are multiples of
10. We know this because they both end
in a zero. Do you notice anything else about
our two additions? They both contain the digits seven
and two. These two calculations seem to be
linked in some way. The first one looks a little bit
easier than the second one. Perhaps we could use it to help us
work out the answer to the second one. Let’s begin then by finding the
answer to seven plus two.
We can start by modeling the number
seven using seven ones cubes, and the two that we’re adding are the same as two more
ones. Let’s start with seven and count on
another two. Seven, eight, nine. We know that seven ones plus two
ones equals nine ones. Seven plus two equals nine. And you know we can use this idea
of seven plus two equals nine to help us solve the second addition. We’ve said already that these two
numbers are multiples of 10. This means we can model them using
10s blocks. They both are number of 10s. 70 is the same as seven 10s. And the 20 that we need to add on
is the same as two more 10s.
And because we know seven plus two
equals nine, we know that seven 10s plus two 10s are going to equal nine 10s. And nine 10s are worth 10, 20, 30,
40, 50, 60, 70, 80, 90. We use the first calculation where
we were adding ones to help us find the total in this second calculation where we
were adding 10s. Seven plus two equals nine. And 70 plus 20 equals 90.
Write the missing number: 10 plus
what equals 50.
To help us understand what we need
to do to find the answer here, we could draw a part–whole model. Let’s fill in the numbers we
know. This is one of those additions
where the missing number is one of the numbers we add. So we already know what the total
is. This means we can fill in the whole
amount, that’s the top number in our part–whole model, as the number 50. We need to add two numbers together
to make 50. And if we look at our calculation,
we already know what one of those numbers is? One of the parts that makes 50 is
10. Now, it’s our job to find out what
the missing part is.
What do we need to add to 10 to
make 50? What do you notice about the
numbers that we know already in this addition. They both end in a zero, don’t
they? This means they’re multiples of
10. We can make them by adding several
10s together. We could even change our part–whole
model to show this. 50 is the same as five 10s. And of course, the number 10 is one
10. Now what do we add to one 10 to
make a total of five 10s? We know that one plus four more
make five. So one 10 plus four more 10s makes
five 10s. And so 10 plus 40 equals 50. Our missing number is 40.
Find the two numbers from 10, 20,
60, and zero which add up to 30.
In this question, we’re looking for
two numbers that we’re going to add together to give us a total of 30. But we can’t just pick any two
numbers. We need to pick two numbers from
the numbers that we’re given. And these are 10, 20, 60, and
zero. Now, there are two ways we could
find the answer here. One of them is a really quick way
that we can find the answer. But we’ll go through the other way
first and we’ll just mention that at the end.
Now, there’s something interesting
about all the numbers in this question. Can you see what it is? Each of the numbers, so that’s the
two numbers that we need to add together and also the total that we’re trying to
make, ends in a zero. This means they’re all what we call
multiples of 10. We can make each number out of an
amount of 10s. 10 is the same as one lot of
10. We can make 20 from two lots of
10. 60 is the same as six lots of
10. Zero is no lots of 10. And finally, the number we’re
making, 30, is worth three lots of 10.
This is where we should start
really. We need to try to make three lots
of 10. Which two numbers can you see that
we can add together to make three lots of 10? We can use a number fact to help us
here. We know that one plus two equals
three. And so we can say one 10 plus two
10s equals three 10s. Or to say it another way, 10 plus
20 equals 30.
Now, we did say at the start of
this question that there was a really quick way to find the answer. The answer had to be 10 and 20
really. We know that if one of our numbers
was zero and we wanted to make 30, then the other number would have to be 30. And we didn’t have 30 as one of our
choices. So we could have crossed through
the number zero before we started. And also, if we’re making the
number 30, another 60 is too large. So we could also have crossed
through the number 60 before we started too. Sometimes there’s more than one way
to find an answer. The two numbers which add up to 30
are 10 and 20.
So what have we learned in this
video? Firstly, we’ve learned what a
multiple of 10 is, and also how to describe multiples of 10 as a number of 10s. And then we’ve learned how to add
two multiples of 10 together and then model this using place-value equipment.