When we want to add 2 big numbers quickly, and we don't care about the exact answer, we can estimate the sum.
Estimating a sum is done by rounding the addends first and then adding them together.
An estimate is not the exact sum, but it's close enough.
Let's try an example.
Estimate the sum of 487 and 306.
We start by rounding each addend.
Rounding is finding a simpler number that's still close to what it was.
When you round, first pick a target digit to round to.
Then look at the digit to the right to know what to do next.
If the digit to the right of the target digit is less than 5, you round down.
If the digit to the right of the target digit is 5 or more, you round up.
Tip:
When estimating sums, round each addend, or number, to its highest possible place value. This is the place value of the leftmost digit.
What's the highest place value in the numbers 487 and 306? 🤔
Yes, it's the Hundreds digit in both numbers.
What's 487 rounded to the nearest hundred?
That's right, 500.
What's 306 rounded to the nearest hundred?
Yes, 300.
After rounding, we add the rounded addends to get an estimate.
We just add 5 + 3, which gives us 8. Then we added 2 zeros at the end.
Using estimation, we see that the sum of 487 and 306 is about 800.
This is how we write the results of an addition with an estimated sum.
487 + 306 ≈ 800
We don't use the equal sign in estimation. Instead, we use the approximate symbol that is made of two squiggly lines (≈). It tells us that something is almost equal to something else.
225 + 43 + 7,158 ≈ ?
Let's estimate the sum.
To estimate, we start by rounding each number to its highest place value.
What's 225 rounded to the nearest hundreds place?
You got it, 200. ✅
What's 43 rounded to the nearest tens place?
Yes, 40. ✅
What's 7,158 rounded to the nearest thousands place?
That's right, 7,000. ✅
Now, we can estimate the sum by adding the rounded values.
Let's rewrite the problem in column form.
The sum of 225, 43, and 7,158 is about 7,240. The answer isn't exact, but it's close.
225 + 43 + 7,158 ≈ 7,240
Great job.
Now, practice estimating sums on your own.