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

Fruits and Milk


NUMBER OF CALORIES IN SOME COMMON FOODS
Apple = 70Milk (1 cup) = 148Potato chips (10 chips) = 240
Banana = 97Malted milk (1 cup) = 276Steamed rice (1 cup) = 120
Carrot = 28Yogurt (1 cup) = 237Fried 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.