Decimal place value
Multiply one-digit numbers with up to two decimal places by whole numbers (e.g. 3.47 × 6)
What a learner can do afterwards
- Correctly compute 4.56 × 7 using a written method
- Use place-value reasoning to explain why 3.2 × 5 = 16.0
- Model a decimal multiplication using an area diagram or expanded form
The lesson
You already know how to do long multiplication with whole numbers. Decimals use the exact same steps, plus one extra move at the end. Multiply the digits as if the decimal point were not even there. Then count how many digits come after the point in the decimal number, and put the point back that many places from the right in your answer.
Work out 4.56 × 7. First, multiply as whole numbers: 456 × 7 = 3192. Now count the digits after the point in 4.56: there are 2 (the 5 and the 6). Put the point 2 places from the right in 3192, and you get 31.92.
A common slip is lining up the decimal points like you would for adding. Don't do that here. Multiply first, then count the decimal digits in the question to place the point in the answer.
Multiply the digits like whole numbers, then move the decimal point in from the right by the same number of digits that followed the point in the question.
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.