The big idea
Decimals work just like whole numbers. The only extra job is to keep the decimal point in the right place. The point separates whole numbers from parts of a whole.
Let's explore
Adding and taking away. Line up the points, one under the other. Then work column by column, just like normal. If one number is shorter, add zeros to fill the gap. A zero at the end after the point does not change the value. So 2.8 is the same as 2.80.
3.45
+ 2.80
------
6.25
Multiplying and dividing by 10, 100 and 1000. The digits slide.
- 3.7 × 10 = 37 (one place left)
- 3.7 × 100 = 370 (two places left)
- 5.6 ÷ 10 = 0.56 (one place right)