# Rates

A rate is a relationship between two quantities of different types. Common examples are:

• Grocery price - dollars per kilogram (\$/kg)
• Exercise energy - kilojoules per minute (kJ/min)
• Speed - kilometres per hour (km/h), metres per second (m/s)

The word "per" and the slash symbol / mean "divide".

## Example One - Cost of Strawberries

Strawberries cost \$4.50 for 5 kg. What is the price per kilogram?

 Price per kilogram = \$4.50 = \$0.90/kg 5 kg

## Example Two - Best Buy of Sweets

Which is the better buy of sweets - 5 kilograms of jelly babies for \$28.40 or 500 grams of jelly beans for \$11.45?

 Jelly babies' price per kg = \$28.40 = \$5.68/kg 5 kg
 Jelly beans' price per kg = \$11.45 = \$22.90/kg 0.5 kg

Jelly babies are cheaper.

## Did You Know This...?

Many sweets, jellies, sauces and medicine capsules contain Gelatine, a colourless and flavourless powder extracted from collagen in slaughtered pig hides. Is that sweet to you?

## Did You Know That...?

If the average frog has a mass of 0.5 kg, less than 0.05 grams of Fluoride (more accurately, sodium monofluorophosphate) in toothpaste would kill it. Do you consider how often you clean your teeth in a year and the possible impact on other living creatures in the environment?

## Questions - Exercise Energy

 EXERCISE ENERGY ( kJ / kg / 30 minutes)This is measured in kilojoules of energy burnt for every kilogram of body mass in each 1⁄2 hour. Aerobics = 10.6 Jogging = 17.0 Basketball = 20.5 Mountain-biking = 18.5 Canoeing = 6.5 Swimming = 15.7 Dance = 9.2 Tennis = 14.7 Golf = 8.6 Walking (fast) = 9.3

Q1. I am 60kg. When I do my favourite exercise for 1 hour, I burn off 780 kilojoules. What sport is this?

Q2. Mountain-biking Bob, whose mass is 72 kg, finishes a race in 30 minutes. How much energy did he burn off?

Q3. A chocolate bar contains 1722 kilojoules of energy. How long must Tall Tess, a keen lean 56 kg basketball player, play to burn this off?