Estimation Strategies and Rounding Rules
When rounding, you will need to know which place value you will need to round to. When you find out which place to round to you can determine whether or not to round up or down based on the number to the right of the target place value. If the number is from 0-4, round down. If the number to the right of the target place value is 5-9, round up. For example, to round 123.234 to the nearest hundredth, you must look to the thousandth place to determine whether or not to round up or down. Because the thousandths place is 4, you will round down, giving you an answer of 123.23.
Rounding and estimation are helpful tools that allow us to simplify numbers so that they are easier for our brains to work with.
When rounding, the first thing you need to decide is your “round- off” digit. For instance, let’s say you are asked to round a number to the nearest hundreds place. The round off digit is the digit in the hundreds place. The round off digit is always the place value of the number that you are rounding to.
Once you know your round off number, you then need to look at the very next number to the right of the round off digit. If that number is less than 5, then the round off digit stays the same and you change everything to the right of it to zeros. If that number is 5 or greater, add one to the round off digit and change everything to the right of it to zeros.
Let’s look at an example:
Round 7,890.45 to the nearest hundred.
So, we need to find our round-off digit. Which is the number in the hundreds place, so “8.” Now, look at the digit one place to the right, “9.” Nine is more than five, so our round-off digit “8” needs to be rounded up. So, we add one to the round-off digit, and change all the numbers to the right to zeros. Since, everything to the right are zero’s anyways you can change eliminate the decimal from your number.
So, our answer is 7,900.
Let’s look at another example.
Round 37.9824 to the nearest thousandth.
Remember thousandth is not the same as thousand. Thousandth lets us know that we are dealing with a decimal number, and thousand lets us know we are dealing with a whole number.
So, let’s find our round-off digit, which is “2.” Now, look at the number to the right of the round-off digit, “4.” Since four is less than 5, the round-off digit stays the same, and everything to the right of it can be dropped.
So, our answer is 37.982.
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