Video Transcript
Estimate the sum by rounding each
number to the nearest ten: 54 plus 28 equals what.
In this question, we’re given two
two-digit numbers, and they’ve been written as a column addition. Now, you might be forgiven for
looking at this and thinking, “Oh! I’ve gotta work out some column
addition here.” But you know, we don’t have to do
this. We are not asked to find the exact
sum of these numbers. We need to estimate it. In other words, we need to find a
total that’s around about the right answer — doesn’t have to be exact though. And we’re told that we need to
estimate the sum by rounding each number to the nearest ten.
Let’s start by looking at our first
number 54. Can you think which two multiples
of 10 54 comes between? It’s between 50 and 60. But which one of these two
multiples of 10 are we going to round it to? Which is it nearest to? To help us, we can look at the ones
digit of this number. It’s a four. Do you remember the rule for
rounding? If the ones digit of a number is a
four or less, we round down. And this ones digit is a four, so
we’re going to round down to 50. So let’s erase 60 from the screen,
and we’re going to change then 54 into 50. By rounding this first number to
the nearest ten, we’ve made it a lot easier to work with.
Our second number 28 comes
somewhere between 20 and 30. But again, we need to ask
ourselves, “which is the nearest ten?” This time, the ones digit in our
number is an eight. And if numbers with a four or less
in the ones place, we round down. This means that numbers with a five
or more round up. Of course, eight is more than five,
isn’t it? So we need to round this number
up. 28 is nearest to 30. So instead of thinking of our
calculation as a column addition that shows 54 plus 28, we’ve made the numbers so
simple. We don’t even need to think of it
as a column addition at all. We just need to find the answer to
50 plus 30, or if we think of it another way, five 10s plus three 10s.
But we know that five plus three is
the same as eight. So five 10s plus three 10s will be
eight 10s, and 50 plus 30 is the same as 80. And you know the interesting thing
about our estimate is how close it is to the actual answer. And if we’d have worked out the
exact answer at the column addition, we’d have got the answer 82. So our estimation is really close,
isn’t it? It’s a good method to use. We’ve estimated the sum of 54 and
28 by rounding each number to the nearest ten. Our estimation is 80.
