Ray's New Practical Arithmetic · Art. 144 · Unit 12: Decimal Fractions (continued)
68. Subtracting decimals
Goal: Your child subtracts decimals with the points lined up, filling empty places with ciphers.
You'll need
- Paper and pencil, or a slate or small whiteboard
What's in the book
Page 179 defines Subtraction of Decimals (Art. 144) and works one example. Page 180 gives the rule and problems 2–14, each with its answer printed.
The lesson, step by step
Warm-up
Ask: If you have $5 and spend $1.25, how much is left? Talk through borrowing a dollar as 100 cents.
The idea
Subtraction of Decimals is the process of finding the difference between two decimal numbers.
The rule works like addition: line up the points, then
2. Subtract as in simple numbers.
and put the point in the remainder under the column of points.
Work the book's example
From 729.835 subtract 461.5738.
The top number has only three decimal places. Write it as 729.8350 so every column has a digit. The remainder is 268.2612.
Guided practice
Do these two together:
From 97.5168 subtract 38.25942.
From 3 subtract .00003.
The second needs 3 written as 3.00000. Borrowing across all those zeros is good practice.
Independent practice and check
Your child works the rest of problems 2–14 on page 180. 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
Subtract aloud: 1 − .25; 2 − .5; .9 − .45; 10 − 9.99.
Tip
Most mistakes come from lining up the right-hand digits instead of the decimal points. Have your child draw a light vertical line through the points before starting.