Video Transcript
Find the value of 90 multiplied by
three.
In this question, we’re being asked
to multiply a multiple of 10 by a one-digit number. We know 90 is a multiple of 10
because it ends in a zero. That’s always a good way to spot
them. And multiples of 10 are what we get
when we multiply a number by 10. And 90 is nine 10s, isn’t it? This is a fact that we can use to
help us find out the answer. And we can use place value blocks
to help see how. So firstly, if we think of 90 as
nine 10s, we can write our calculation as nine 10s multiplied by three, or in other
words three lots of nine 10s.
Let’s model these using place value
blocks. One, two, three lots of nine
10s. How many 10s blocks have we got
here? Let’s use our knowledge of the
three times table to help. Here we’ve got three lots of nine
ones. And if we know how many ones blocks
we’ve got, we also know how many 10s blocks we’ve got. Let’s count in nines three
times. Nine, 18, 27. We’ve got 27 ones blocks. And because we’ve got nine 10s
multiplied by three, we’ve also got 27 10s blocks.
But they’re not worth 27, are
they? Remember, each of our 10s blocks is
worth 10. And so to solve this problem, we
need to take our answer to nine times three and multiply it by 10. The diagram on the right is 10
times as large as the one on the left because each of the blocks we’ve used is worth
10 times as much. So what’s 27 multiplied by 10? Well, we know when we multiply any
number by 10, each of its digits becomes 10 times as large. And so it shifts one place to the
left. Instead of the two in 27 being
worth two 10s, it shifts one place to the left and is now worth two 100s. Instead of the seven in 27 being
worth seven ones, that also shifts one place to the left and is now worth seven
10s. And we need to include a zero to
show that we’ve got an empty ones column. 27 multiplied by 10 equals 270.
We’ve found the answer to 90
multiplied by three by thinking of the calculation as nine 10s multiplied by
three. We know that nine 10s are 10 times
as large as nine ones. So first, we found the answer to
nine ones multiplied by three, which was a lot easier. Nine times three equals 27. And then we just found the number
that was 10 times as large as this by multiplying 27 by 10. The value of 90 times three equals
270.
