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Lesson 36: Estimating by Rounding
This lesson talks more about how to find answers by rounding. If a test question uses a phrase similar to "about how many," that's usually a clue that they expect you to do some rounding. Sometimes it's up to you to figure out how and what to round. This lesson explains more.
Don't forget to watch the embedded video clip for this lesson at the bottom of the page. Please be sure to read the embedded video information and disclaimer.
Here is a typical problem involving rounding. Joe has 316 baseball cards and Mary has 582 baseball cards. About how many do they have combined?
When we see a problem like this, it means we need to do some rounding. Review Lesson 12 for a review of the basics of rounding. In this problem, what makes the most sense is to round to the nearest hundred. Both numbers are fairly close to a multiple of a hundred, and chances are if this is a multiple choice problem, each answer will be a multiple of one hundred.
To round the first number to the nearest hundred, we'll look at the digit in the tens place, which is to the right of the hundreds place. Since it's 4 or lower, we'll round the number down to 300. The second number has an 8 in the tens place. Since it's 5 or higher, we'll round the number up to 600. Adding them together we get an approximate answer of 900. It's not the exact sum, but we were only asked for an answer that is about right.
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