Ray's New Practical Arithmetic · Art. 143 · Unit 12: Decimal Fractions (continued)
67. Adding decimals
Goal: Your child adds decimals by lining up the decimal points, and can write decimals given in words.
You'll need
- Paper and pencil, or a slate or small whiteboard
What's in the book
Page 178 defines Addition of Decimals (Art. 143), works one example with the rule, and starts the problems. Page 179 continues to problem 12, some written in words ("4 and 8 tenths"). All answers are printed.
The lesson, step by step
Warm-up
Ask your child to add $3.25 and $0.80 in their head. Then talk about why the cents had to be lined up with the cents.
The idea
Read:
Addition of Decimals is the process of finding the sum of two or more decimal numbers.
The rule has three steps: write the numbers so the decimal points and figures of the same order stand in the same column; add as in simple numbers; put the decimal point in the sum under the column of points.
Work the book's example
Add 375.83; 49.627; 5842.1963; 813.9762.
Write them in a column with the points in a straight line. Fill empty places with zeros if that helps. The sum is 7081.6295.
Guided practice
Do these two together:
Add 37.1065; 432.07; 4.20733; 11.706.
Add 21.611; 6888.32; 3.4167.
For the second, check that the answer (6913.3477) has four decimal places, as many as the longest number.
Word problems
Problems 3 and 7–12 are written in words, like "43 and 31 hundredths". Have your child write each one in figures first and check it with you before adding. The hard part is the place: "19 thousandths" is .019, not .19.
Independent practice and check
Your child works the rest of the problems on pages 178–179. The book prints the answer at the end of each problem line. Have your child cover the answers with a strip of paper, work the problem, then slide the strip down to check.
Mental-math wrap-up
Add aloud: .5 + .25; 1.2 + .08; .75 + .75; 2.5 + 3.75.