Lesson 3 of 15
Times a whole number
Show on a big screenYou cut three pieces of tape, 1.2 m each. How much tape is that?
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Step 1 of 4
- Start at 0 on the line.
- Hop 1.2. Land on 1.2.
- Hop 1.2 again. Land on 2.4.
- Hop 1.2 again. Land on 3.6.
The rule
Three pieces of 1.2 m make 3.6 m of tape.
Adding works: 1.2 + 1.2 + 1.2 = 3.6. Adding is slow for big counts.
Remove the point and multiply the digits as whole numbers. Then give the answer the same number of places. Places are the digits after the point.
1.2 becomes 12. 12 × 3 = 36. 1.2 has one place, so the answer has one place: 3.6.
Why? 1.2 is 12 tenths. 36 tenths is 3.6.
| Sum | Digits | Places | Answer |
|---|---|---|---|
| 1.2 × 3 | 12 × 3 = 36 | 1 | 3.6 |
| 2.5 × 4 | 25 × 4 = 100 | 1 | 10 |
| 0.35 × 6 | 35 × 6 = 210 | 2 | 2.1 |
Watch out!
Do not multiply the two sides of the point apart. 3.75 × 2 is 7.5. Splitting gives 3 × 2 = 6 and 75 × 2 = 150. Those do not join into 7.5.
Try the lab to finish this lesson.