Multiplying and dividing (age 10+)
Multiply and divide numbers by 10, 100, and 1000 giving answers up to three decimal places, understanding that digits shift position in the place-value chart
What a learner can do afterwards
- Compute 3.456 × 100 = 345.6 correctly
- Compute 45.2 ÷ 1000 = 0.0452 correctly
- Explain why multiplying by 10 shifts each digit one place to the left
The lesson
You already know how to multiply, divide, and read decimal numbers. Here is a shortcut for 10, 100, and 1000: instead of working out the whole calculation, you shift every digit to a new place in the place-value chart. Multiply by 10, and digits shift one place left. Multiply by 100, two places left. Multiply by 1000, three places left.
To work out 3.456 x 100, shift every digit two places left, since 100 has two zeros. The 3 moves from the ones place to the hundreds place: 3.456 becomes 345.6.
To work out 45.2 divided by 1000, shift every digit three places right, since 1000 has three zeros. There are not enough digits, so you fill the empty places with zeros: 45.2 becomes 0.0452.
Count the zeros in 10, 100, or 1000. That tells you how many places to shift. Multiplying shifts digits left and makes the number bigger. Dividing shifts digits right and makes the number smaller.
Count the zeros, then shift every digit that many places: left to multiply, right to divide.
Watch it
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.