Multiplication of decimals

Multiplying one decimal by another can be done by converting the decimals to fractions. Then multiply the fractions in the usual way and converted back to decimals.

For example:

\begin{align}0.6×0.4&=\dfrac{6}{10}×\dfrac{4}{10}\\\\ &=\dfrac{24}{100}\\\\ &=0.24\end{align}

A second widely used method is as follows.

Step 1:

Ignore the decimal points and multiply the factors as if they were whole numbers.

Step 2:

Insert a decimal point in the product so that the total number of decimal places is the same on both sides of the equation.

For example:

2.5 × 0.06

25 × 6 = 150

Step 2:

2.5 × 0.06 = 0.150 = 0.15

Example 1

Multiply 0.03 × 0.18 using both methods as described above.

Solution

\begin{align}0.03×0.18&=\dfrac{3}{100}×\dfrac{18}{100}\\\\ &=\dfrac{54}{10000}\\\\ &=0.0054\end{align}
\begin{align}0.03 × 0.18 = 0.0054\hspace{4mm}(3 × 18 = 54.\ \text{There are 4 decimal places}.)\end{align}

Summary

• Multiply decimals by converting each decimal to a fraction, multiplying the fractions (without cancelling) and converting the result back to a decimal.

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