# Multiply Decimals

## Multiplying Decimals by Powers of Ten:

If you **multiply** a decimal number **by 10**, the decimal point moves **1 place** to make the number **larger**

(for example 3.456 × 10 = 34.56).

If you **multiply** a decimal number **by 100**, the decimal point moves **2 places** to make the number **larger**

(for example 3.456 × 100 = 345.6).

If you **multiply** a decimal number **by 1000**, the decimal point moves **3 places** to make the number **larger**

(for example 3.456 × 1000 = 3456).

## Dividing Decimals by Powers of Ten:

If you **divide** a decimal number **by 10**, the decimal point moves **1 place** to make the number **smaller**

(for example 2345.6 ÷ 10 = 234.56).

If you **divide** a decimal number **by 100**, the decimal point moves **2 places** to make the number **smaller**

(for example 2345.6 ÷ 100 = 23.456).

If you **divide** a decimal number **by 100**, the decimal point moves **3 places** to make the number **smaller**

(for example 2345.6 ÷ 1000 = 2.3456).

## Questions

**Q1.** 57.8 × 100
**Q2.** 45.9 × 1000
**Q3.** 0.087 × 10
**Q4.** 569.3 ÷ 10
**Q5.** 0.078 ÷ 100

**Answers**
**A1.** 5780
**A2.** 45900
**A3.** 0.87
**A4.** 56.93
**A5.** 0.00078

## Multiplying Decimals by Other Numbers:

## Example One - Cost of Downloading Music

Jesse J's latest album (with 17 tracks) can be downloaded as iTunes at $0.15 per track. How much does it cost to download the whole album?

*(Multiply these numbers as if they had no decimal point first. When multiplying decimals, don't line up the columns. Then place the decimal
point back at the end. The total number of decimal places that you start with is the total number of decimal places that will be in the final answer.)*

**Answer:**

$ 0.15 |

× 17 |

$ 2.55 |

## Example Two - Fuel Consumption

An old (but very cool) Porsche 911 car has a fuel consumption of 6.7 litres per 100 kilometres. (This means that for every 100 km it travels, it uses 6.7 litres of fuel.) If fuel costs 198 cents per litre, what is the fuel cost to travel 1000 kilometres from Berlin to Paris?

**Answer:**

1000 ÷ 100 × 6.7 × $1.98 = $132.66

## Questions - Food Energy

NUMBER OF CALORIES IN SOME COMMON FOODS | ||

Apple = 70 | Milk (1 cup) = 148 | Potato chips (10 chips) = 240 |

Banana = 97 | Malted milk (1 cup) = 276 | Steamed rice (1 cup) = 120 |

Carrot = 28 | Yogurt (1 cup) = 237 | Fried rice (1 cup) = 267 |

To convert calories to kilojoules, multiply by 4.2 |

Refer to the food energy table above. What is the number of kilojoules of energy in:

(a) 1 cup of milk and 1 banana

(b) 1 steamed rice and 1 malted milk?

**Answers**

(a) 1029 kJ

(b) 1663.2 kJ

## Did You Know That...?

Super-bananas which are **Genetically Modified bananas** enriched with Vitamin A have been successfully grown in Australia with generous
funding from Bill Gates, the Microsoft company founder. If successful, they may remedy the Vitamin A deficiency experienced by many
African people whose root vegetable diet is rich in carbohydrate but low in Vitamin A.