Lesson 4 of 15
Times a decimal
Show on a big screenYou know 13 × 12 = 156. Where does the point go in 1.3 × 1.2?
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Step 1 of 4
- One whole grid, 100 small squares.
- Shade 4 columns. That is 0.4 of the grid.
- Shade 3 rows too. That is 0.3 of the grid.
- The two shadings overlap on 12 squares: 0.4 × 0.3 = 0.12.
The rule
In 1.3 × 1.2, the point goes after the 1: 1.56.
Multiply the digits as whole numbers. Count the places past the point in both numbers. A place is one digit after the point. Add them.
Give the answer that many places.
0.4 has 1 place. 0.3 has 1 place. That is 2 places. 4 × 3 = 12, so the answer is 0.12.
Back to the hook. 13 × 12 = 156. 1.3 has 1 place and 1.2 has 1 place. That is 2 places. So 1.3 × 1.2 = 1.56.
Why? 0.4 × 0.3 is 4/10 × 3/10. Tops: 4 × 3 = 12. Bottoms: 10 × 10 = 100. So the answer is 12/100, which is 0.12.
Try the lab to finish this lesson.