Multiply a Decimal by a Whole Number

Multiply a Decimal by a Whole Number

When you multiply a decimal by a whole number, you are finding repeated groups of the decimal. The main idea is to multiply as if the decimal point were not there, then place the decimal point in the correct spot at the end.

Method

1. Ignore the decimal point first. Multiply the numbers as whole numbers.
2. Count the decimal places in the decimal factor.
3. Place the decimal point in the product so it has the same total number of decimal places.
4. Write the answer in simplest exact form. Do not round unless told to.

Example idea

If a decimal has 2 decimal places, the product should reflect that same precision. For instance, multiplying by a whole number does not change the number of decimal places in the decimal factor; it only changes the size of the value.

Check your work

• Estimate mentally: the answer should be larger than the decimal if the whole number is greater than 1.
• Recompute by breaking the whole number into parts if needed.
• Make sure the decimal point is placed correctly by counting places from the right.

Common mistake

A frequent error is placing the decimal point too far left or right. If the number looks unreasonable, recheck the decimal placement before finishing.