Math Focus 4
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## CHAPTER 3: ADDITION AND SUBTRACTION

### Lesson 2: Estimating Sums

Goal

Estimate sums in different ways.

Instructions for Use

To use The Addition Machine, scroll down the central list and click on its name under “Skills practice programs, games, and puzzles”. On the new screen that displays, read the instructions and choose Level 3. Estimate first, then solve the problem that appears by adding from left to right, clicking on the empty box right of the equals sign, and typing your answer. Then click on “Check my answer”. If it is right, “Well done!” will display to its right and the score below will show the number value of the answer and “1” under “Correct”. If not, “Try again” will display and the score will be less the number value of the answer and show “1” under “Incorrect”. Correct the answer and click on “Check my answer” again to get back your lost number value. Then begin a new round by clicking on Level 3 again. Your score will go up with every correct answer and down with every incorrect answer that you do not correct before starting another round.

To use Addition of Three-Digit Numbers, read the directions at top and click on “New Problem” at lower right to begin. Type any number between 1 and 999 in the empty box on the left of the “+” sign above and hit “Enter” on your keyboard. The number will appear as base blocks to the left and in place value columns below. Do likewise for the empty box to the right of the “+” sign and watch as your second number appears below the first. If any column has more than 9, click on “Regroup” and watch as the numbers and blocks regroup. Continue regrouping until the column totals are all 9 or less, and then click on “Check”. If you have regrouped all the columns you will see “Great job! Click on New Problem to start over.” If not you will see “Keep going. More regrouping is needed.”

To use any site, read its instructions. Make sure that the “Num Lock” key on your keyboard is on and then double click in the coloured problem area to start the timer. Add the numbers mentally from right to left and type in your answer, starting with the hundreds digit first, by using the number keys at the right of your keyboard. Backspace over any mistyped numbers and correct them, and then press “Enter” on your keyboard. A score sheet at the lower left of the problem area shows your time remaining in seconds and the number of problems you have answered correctly and incorrectly. See how many problems you can get correct in the time available, and then click on the yellow bar below to start a new round.