Adding pieces of one size
Cut a strip into 8 equal parts. Slide 3 parts together. Put 2 more parts next to them. Now there are 5 parts, all the same size.
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The rule
Parts of one size are easy to add. The tops add. The bottom number does not change.
The parts are the same size. So the bottom does not change.
Subtracting works the same way. The tops subtract. The bottom stays.
- A strip cut into 8 equal parts.
- Slide 3 parts together: 3/8.
- Push 2 more against them: 2/8.
- Together: 3/8 + 2/8 = 5/8.
Example
Add the tops: 3 + 2 = 5. Keep the bottom 8. So 3/8 + 2/8 = 5/8.
Subtract the tops: 5 − 3 = 2. So 5/8 − 3/8 = 2/8.
4/8 + 4/8 = 8/8, because 4 + 4 = 8. All 8 parts make one whole.
Parts of different sizes need renaming first. 1/2 = 2/4. Then 2/4 + 1/4 = 3/4.
Watch out!
1/4 + 1/4 is not 2/8. Do not add the bottom numbers. Two quarters make 2/4, which is half.
In the lab, drag loose pieces onto the strip to add. Drag pieces off to subtract.