Adding and Subtracting Numbers up
In this video, we’re going to learn
how to solve problems by deciding whether to add or subtract. And then once we’ve decided, we’re
going to learn how to complete equations by adding and subtracting within 10. Now, if we’ve been given a number
sentence to solve, we might look at the symbol that’s in that number sentence. And it’s gonna tell us what to
do. We might see this symbol, which
shows that we need to add to find the answer. Or we might see this symbol, which
tells us we need to take away or subtract. But what if we’re given a problem
that’s written all in words and we’re not given a number sentence at all? How do we know whether we need to
add to find the answer or take away?
Well, problems where we need to add
to find the answer are often about putting groups or numbers together or adding on
to something. We might come across words like
what is the “total,” how many “altogether,” or any words that’s mean the same as
adding like “combine.”
What about problems where we need
to subtract to find the answer? These are often about taking away a
group or a number from the whole amount and seeing what’s left behind. We need to keep our eyes open for
words like “take away” or questions that ask us how many we’ve got left. Let’s have a go at answering some
word problems now. We’re going to be able to solve
them by either adding or subtracting. And the very first question we need
to ask ourselves each time is, how can we tell whether to add or subtract?
A fisherman caught two fish. Then, he caught one more. Which of these tells you how many
fish he caught in total. Two plus two, two plus one, or two
take away one.
In this question, we’re given a
word problem. To begin with, we’re told that the
fisherman caught two fish. And in the picture, we can see
those two fish. We’re then told that he caught one
more. This is the red fish that we can
see in the picture. We’re then asked to choose an
answer which tells us how many fish the fisherman caught in total.
Now, if we look at our possible
answers that we have to choose from, we can see that two of them are additions and
one of them is a subtraction. So the first question we need to
ask ourselves is, do we need to add to find the answer here? Or do we need to take away? If we look at the words in our word
problem, we can see some clues.
Firstly, there’s the word
“more.” A fisherman caught two fish. Then he caught one more. If we find one more than something,
we need to add to find the answer. The word “total” is another
clue. If we want to find the total of
some numbers, we add them together. So we’re going to need to add to
find the total number of fish the fisherman caught. Let’s cross out the
subtraction. We know this isn’t the right
answer. Now, which of our two possible
additions is correct? Two plus two or two plus one?
Well, we know that the fisherman
caught two fish and one more. And to find one more than two, we
need to add two and one together. Two plus one is one, two,
three. We know the fisherman caught three
fish in total. And of course, we can see that in
the picture. And so the expression that tells us
how many fish he caught in total is two plus one.
Eight children are at a party, but
then two have to leave. Which of these tells you how many
children are left at the party? Eight plus two, six plus two, or
eight take away two.
If we look at our three possible
answers in this word problem, we can see that two of them have addition symbols,
where we need to add to find the answer. And one that has a subtraction
symbol, where we need to take away to find the answer. But which of these is correct? Do we need to add to find the
answer? Or do we need to take away?
Let’s read the word problem more
carefully to try to find out. The first thing we’re told is that
eight children are at a party. And if we count the children in the
picture, we can see that there are eight of them. But the next piece of information
we’re told is that two of the children have to leave. Must be these two children here
because they’re separated from the rest.
Now, our question asks us to find a
way to show how many children are left at the party. So we start off with eight children
at the party. Now, if two of them leave, is the
number eight going to get bigger or smaller? Well, because two children have
left the party, we know that there’ll be less than eight children in the party. The number is going to get
smaller. This means this is a subtraction
Now, out of our three possible
answers, only one of them is a subtraction. So we know the answer must be eight
take away two. Let’s use this and see whether it
does tell us how many children are left at the party. Eight take away two should leave us
with one, two, three, four, five, six children left at the party. And if we count the children in the
picture just to check, we can see that there are six children left at the party.
When we need to find how much of
something is left, we often have to subtract to get there. So the expression that tells us how
many children are left at the party is eight take away two.
You have four blue sweaters and
three red ones. How many sweaters do you have?
If we read this problem quickly, we
might say you’ve just told me the answer. I’ve got four blue sweaters and
three red ones. But if we think more carefully
about the question, we know that when it asks us how many sweaters we have, it’s
talking about the whole amount. How many sweaters do you have in
total? How many altogether?
Now, apart from two numbers, this
problem is written in words. There aren’t any symbols to tell us
whether to add or to take away to find the answer. The first thing we can do then is
to decide. Is this an addition problem or a
Well, the question tells us we have
four blue sweaters and three red ones. You know, it’s very easy to miss
that little three-letter word in between. You have four blue sweaters and
three red ones. In other words, you’ve put together
a group of four blue sweaters and three red ones to make one big group. And when numbers or groups are
combined like this, we know we have to add to find the answer.
Let’s model the problem using
cubes. Four blue cubes to represent four
blue sweaters and then three red cubes. We can write this as an addition
sentence or equation. Four plus three equals what? Let’s count on from the four we
already have to see how many we have altogether. Four, five, six, seven. If you have four blue sweaters and
three red ones, the way to find the answer is to add four and three together. So how many sweaters do you
have? Four plus three equals seven.
This garden had five flowers. Then, Jennifer picked two
flowers. How many flowers are left in the
To solve this problem, it would be
helpful if we had a number sentence or equation. Then we’d know what to do with the
numbers. Should we add them? Or should we take them away? The first thing we need to do is to
read the problem carefully and to think to ourselves, is this an addition or a
The first thing we’re told is that
the garden had five flowers. We could use five counters to
represent our five flowers. Then we’re told that Jennifer
picked two flowers. Now, does this mean that Jennifer
added another two flowers to the five flowers that were already there? If so, we need to work out the
answer to five add two. Or does it mean that Jennifer took
away two flowers from the five that were already there? In which case we need to work out
five subtract two.
Well, of course, if somebody picks
some flowers, the group of flowers is going to get less. We know this because the question
asks us how many flowers are left in the garden. When something’s left behind, it’s
after we’ve taken something away. So to find the answer, we need to
work out five take away two. And if we look at our counters, we
can see that we have three left. Five take away two equals
three. We can use the picture to
check. There are three flowers left
Because this problem was all about
somebody picking flowers out of a garden, we decided it was going to be a
subtraction sentence that we needed to write. The number of flowers that are left
in the garden is the same as five take away two, which equals three.
So what have we learned in this
video? We’ve learned how to solve problems
by doing two things. Firstly, by deciding whether to add
or subtract to find the answer. And then actually finding the
answer by completing equations, by adding or subtracting.