Video Transcript
Estimate 23 plus 36 by rounding the
two numbers to the nearest ten using the number line.
In this question, we’re given two
two-digit numbers to add, 23 and 36. But we’re not asked to find the sum
of these numbers. We’re asked to estimate it, in
other words, come up with a value that’s quite close to the exact amount but not
exactly it. Now if our question just stopped
there, we might look at it and think to ourselves, “Well, how am I going to estimate
this amount? What am I gonna do?” But thankfully, our question
doesn’t stop there, and it goes on to tell us how to estimate the answer. We need to round both of the
numbers to the nearest ten, and we’re given a number line to help us do this.
Have you ever wondered why we would
want to round a number to the nearest ten like this? Well, numbers that are multiples of
10 — and you can see these marked with blue dots on our number line, 20, 30, 40, and
so on — they end in zero. But when it comes to working with
them, and in this case we’re going to add them, we don’t need to think about the
ones digit. They’re already zeros. We just need to add together the
tens. If we want to estimate the sum of
two numbers really quickly, changing them into multiples of 10 is a good thing to
do. It makes the whole thing a lot
easier.
So let’s begin by writing out our
addition as it is at the moment, 23 plus 36. And we’re gonna make this easier to
work out. So we’ll take our first number 23
and we’ll see where it lives on our number line. Can you see it? It’s here, isn’t it? It’s in between 20 and 30. But which of these two multiples of
10 is it nearest to? Where would we round 23 to? Would you round it down to 20 or up
to 30? You could probably see just by
drawing those dotted lines which multiple of 10 it’s nearest to. But it’s often useful just to mark
in the halfway point between 20 and 30, and that’s 25. Can you see that 23 is less than
25? It’s nearest to 20, isn’t it?
So the first thing we can do to our
calculation is to change 23 to a much easier number to deal with, 20. It’s not gonna give us the exact
amount anymore, is it? But it’s gonna give us a good quick
estimate. Our second number is 36. Can you see where this is on our
number line? It’s here; it’s in between 30 and
40. But again, which of these two
multiples of 10 is it nearest to? Let’s mark in the halfway point
again. Halfway between 30 and 40 is here,
35. And we know that 36 is just larger
than 35, isn’t it? We’re going to need to round 36 up
to 40. So let’s change the second number
in our addition, 40.
Now we can find our estimate. We need to add together 20 and
40. Now, we know that two plus four
equals six, and we can use this easy fact to help us now. Two 10s or 20 plus four 10s or 40
is going to give us six 10s or 60. We rounded both the numbers in the
calculation 23 plus 36 to the nearest ten. And this didn’t give us an exact
answer, but it did give us a good estimate. Did you know the actual answer to
this addition is 59? So you can see how close our
estimate is. Our estimated answer to 23 plus 36
is 60.
