Video Transcript
Look how Lily estimated the product
by rounding the three-digit number to the nearest hundred. 562 times six is close to 600 times
six. Six hundreds multiplied by six
equals 36 hundreds. I estimate that 562 times six will
be close to 3600. Use her method to estimate 792
multiplied by five, and then use her method to estimate 253 multiplied by nine.
Lily is multiplying a three-digit
number by a single digit. And, if you notice, that’s what the
two calculations that we’ve been asked to estimate have in common. They’re also a three-digit number
multiplied by a single digit. We’re told that Lily doesn’t work
out the answer exactly. But, what she does is to estimate
the product. And, what she does is to round the
three-digit number to the nearest hundred. She knows that 562 is close to
600. And so, she round it up to 600. Then we can see, because she knows
that six times six equals 36, she also knows that six hundreds times six equals 36
hundreds. So, her estimation for the answer
to 562 multiplied by six is close to 36 hundreds or 3600. So, let’s use her method now to
estimate the answer to our two calculations.
Firstly, 792 multiplied by
five. Firstly, we need to round 792 to
the nearest hundred. We know that the two multiples of
100 that it’s between are 700 and 800. The halfway point between these two
numbers is 750. And, we know that 792 is a lot
nearer 800 than this point, probably about here. In fact, it’s only eight away from
800. So, we can say, just like Lily, 792
multiplied by five is close to 800 multiplied by five.
Now, we can use our knowledge of
times tables to help. We know 10 times five is 50. So, eight times five is two lots of
five less than 50 or 10 less than 50, which is 40. So, eight hundreds multiplied by
five equals 40 hundreds. And, we know that 40 hundreds are
the same as 4000. So, we can estimate that 792
multiplied by five will be close to 4000.
Now, let’s use her method again
this time to estimate 253 multiplied by nine. We know that the two multiples of
100 that 253 lies between are 200 and 300. And again, it’s useful to think
about where the halfway point is, which is 250. 253 is very close to the halfway
point, but we can see that it’s slightly more than it, perhaps about there. So, even though it’s not as close
to the larger 100 as before, it’s still closer to 300 than to 200. And so, we can say that 253
multiplied by nine is close to 300 multiplied by nine.
Which times tables factor we’re
going to use to help us here? Well, if we know what three times
nine is, we can use it to help us find out what three hundreds times nine is. Again, knowing what a number times
10 is will help us here. Three times 10 is 30. So, three times nine is three less
than 30. The answer is 27. So, three hundreds times nine
equals 27 hundreds. We know 27 hundreds is another way
of saying 2700. So, we can say that 253 multiplied
by nine will be close to 2700.
We used Lily’s method to estimate
the answer to both of our calculations. First, we rounded each number to
the nearest hundred. This gave us a new calculation that
was a lot easier to work out. By writing each calculation as a
number of hundreds multiplied by a single digit, we could just use the times tables
fact to help. And so, we can estimate that 792
multiplied by five will be close to 4000, and 253 multiplied by nine will be close
to 2700.
