Estimating
Estimating is an important skill when adding, subtracting, multiplying or dividing. We can use it before a calculation to give us an idea of the approximate sum, difference, product or quotient. We can use it after a calculation to check our answer.

Last week we discussed several addition and subtraction estimation strategies.
A key skill in estimating is ROUNDING. We want to round the numbers in a way that the calculation may be done mentally. If after you round you have to perform pencil and paper regrouping, then your estimation strategy is not terribly effective. Zeros are are friends!

Example 5855 - 2374

Strategy #1: Round both numbers.
  6000
-2000
 4000

Strategy #2: Keep the first number the same, round the second number to make subtraction easier.
  5855
-2000
 3855

Strategy #3: Look only at the place values with the greatest value. In this case the thousands and hundreds.
  59 hundreds
- 24 hundreds
 35 hundreds or 3500

We round 5855 to 5900 because the 5 in the tens place tells us to round up.
We round 2374 to 2400 because the 7 in the tens place tells us to round up.

Generally to decide whether to round up or down we look at the place to the right of the place we are rounding to. If there is 0, 1, 2, 3 or 4 next door, round down (keep the digit the same). If there is 5, 6, 7, 8 or 9 next door, round up (add 1 to the place you are rounding to).

The above are not rules carved in stone. They are basic guidelines. Sometimes you may want to round one number up and one number down to get a more accurate estimate. This is perfectly acceptable AS LONG AS YOU CAN EXPLAIN YOUR THINKING.

For the upcoming test, you must be able to round several different ways.

Question...... Which estimation is the most accurate?