Video Transcript
Multiplying Numbers Using Bar
Models
In this video, we will learn how to
draw bar models and write equations to represent one-step multiplication problems
with numbers up to 100.
In this problem, we have to
calculate the total number of fish. We know there are four fish
tanks. We also know that each fish tank
contains four fish. One way we could calculate the
answer would be to add together our four lots of four: four plus four plus four plus
four. One way to work out the answer
would be to skip count by four. Four, eight, 12, 16. We skip counted by four four
times. Four plus four plus four plus four
equals 16. Repeated addition or skip counting
by fours is the same as multiplying. There are four fish tanks. Each fish tank contains four
fish. Four lots of four or four times
four equals 16.
In this video, we’re going to learn
how to solve multiplication problems using bar models and multiplication
equations. Let’s try one more example. Gel pens are sold in packs of
three. If this boy buys five packs of gel
pens, how many pens will he have? The first thing we can do to help
solve this problem is to sketch a bar model. Our bar model has five parts or
five squares to represent the five packs of gel pens. And we know that each pack contains
three gel pens. One way we could solve the problem
is to use repeated addition: three plus three plus three plus three plus three. But we could also solve the problem
using multiplication. If we have five groups of three, we
have to add together five lots of three. We could write this as five times
three.
To calculate the answer, we could
count in threes five times. Three, six, nine, 12, 15. Five times three is 15. The boy has 15 gel pens
altogether. First, we read the problem and drew
a bar model. Then we used the bar model to help
us write our multiplication equation. Then we calculated the answer. Let’s try some questions now where
we have to solve one-step multiplication problems using bar models and
multiplication equations.
Anthony has three boxes of
plates. Each box contains five plates. How many plates are there in
total?
In this word problem, we’re being
asked to calculate the total number of plates Anthony has. We can see that Antony has already
drawn a bar model to represent the problem. Each of the different colored bars
represents one box of plates. There are three different colored
bars to represent the three boxes. And Anthony has drawn five squares
inside each bar because each of his three boxes contains five plates. Anthony has also written five plus
five plus five equals what. So we need to calculate three lots
of five.
Adding three fives is the same as
multiplying five by three. We can calculate five plus five
plus five or five times three by counting forward in fives three times along our
number line. Five, 10, 15. Five plus five plus five or five
times three equals 15. If Anthony has three boxes of
plates and each box contains five plates, then he has 15 plates in total.
Elizabeth has three bars of
chocolate. Each bar contains two pieces of
chocolate. Write an addition sentence to show
how many pieces of chocolate there are.
In this word problem, we’re being
asked to write an addition sentence to show how many pieces of chocolate Elizabeth
has. Elizabeth has already drawn a bar
model. Her bar model has three parts to
represent the three bars of chocolate. And the question tells us that each
of her bars contains two pieces of chocolate. So we can write a number two in
each of the bars.
Drawing the bar model in this way
helps us to work out our addition sentence. We need to add three lots of two:
two plus two plus two. Adding together three twos is the
same as multiplying two three times. Let’s count in twos along our
number line three times: two, four, six. Two plus two plus two or three
times two equals six. If Elizabeth has three bars of
chocolate and each bar contains two pieces of chocolate, then she has six pieces of
chocolate altogether.
There are four teams in a race. Each team has three people in
it. Anthony made this model. How many people are there?
In this problem, we have to work
out the total number of people in the race. Antony has drawn this bar
model. He’s drawn four bars to represent
the four teams. The question tells us that each
team has three people in it. If we write the number three next
to each bar, now we can see how to calculate the answer.
We could add together our four
threes, three plus three plus three plus three, or multiply three by four. And we can add our four threes or
multiply three four times by counting forward in threes four times along our number
line. One three is three, two threes are
six, three threes are nine, and four threes are 12. Three plus three plus three plus
three or four times three equals 12. If there are four teams in a race
and each team has three people in it, there are 12 people altogether.
David bought three boxes of
pencils. Each box has 10 pencils. How many pencils does David have in
total?
In this word problem, we’re being
asked to work out the total number of pencils that David has. We know that he bought three boxes
of pencils and each box contains 10 pencils. Let’s sketch a bar model to help us
think about how to calculate the answer. Our bar model has three parts. This is because David bought three
boxes of pencils. And we know that each box has 10
pencils inside. We’ve got three lots of 10. So we can multiply 10 by three to
find the answer. We just need to count in tens three
times: 10, 20, 30. 10 times three is 30. If David bought three boxes of
pencils and each box contains 10 pencils, he will have 30 pencils in total.
What have we learned in this
video? We have learned how to solve
one-step multiplication problems using bar models.
