## The Big Idea:

To multiplying decimals, just ignore the decimal and multiply like normal. Then re-insert the decimal in the correct spot.

1. Multiply the numbers just as if they were whole numbers.

(do not align the decimal points)

2. Place the decimal

**a. Using a reasonable estimate method:**

*(one of the most common errors made in multiplying decimal numbers is in misplacing the decimal point in the answer. Determining what a reasonable answer would be before multiplying usually prevents this error.)*

Estimate the product then use that estimation to place the decimal point in the answer.

Example: | $34.5∗20.5$ |

$345∗205=70725$ | |

Think: | $20$ times $35$ would be $700$. |

The answer should be around 700: | $34.5∗20.5=707.25$ |

**b. Counting Decimal Places method:**

• Count the decimal places to the right of the decimal points in each of the original numbers.

• Then put the decimal point in the answer – it will have as many decimal places as the two original numbers combined.

• Start at the right-most digit of the product. Move the decimal point to the left the necessary number of decimal places.

Some times you need to add zeros to the left of the number:

Example: | $0.03 ∗ 1.1$ | (total of 3 decimal places in the product) |

$3 ∗ 11 = 33$ | ||

$0.03 ∗ 1.1 = 0.033$ |