Let's review how to identify and convert rates, as well as using rates to make useful comparisons. Recall the following definition and distinction between a rate and a ratio:
- A ratio is a comparison of the relative size of two or more quantities. For example, if a bag of marbles contains $3$3 blue marbles and $2$2 red marbles, we can describe the comparison between the number of red and blue marbles as a ratio like red:blue = $2:3$2:3.
- A rate is a ratio between two quantities that are measured in different units. For example, the rate a tap leaks may be $30$30 mL every $5$5 minutes. Rates are often expressed as unitary rates (or a unit rate) where the second quantity in the rate has a measure of $1$1. The unitary rate for the leaking tap would be $6$6 mL for every $1$1 minute. To abbreviate this we drop the $1$1 and use the word per or the notation '/' to separate the two units, thus we could write $6$6 mL/min. This compound unit, mL/min, represents the division of one measurement by another to obtain the rate.
Common examples of rates include:
- Speeds, for example $60$60 km/h, $20$20 m/s, $400$400 m every $5$5 minutes
- Growth rates, for example $20$20 cm/year, $15$15 mm/month, $2$2 kg per week
- Cost of groceries, for example $\$5$$5 per kg, $\$1.20$$1.20 per $100$100 g, $\$10$$10 per box
Simplifying and converting rates
A rate is usually considered simplified when it is represented as a unit rate. Remember that a unit rate is a rate where the second quantity is just $1$1 of the unit prescribed. To calculate a rate, divide one quantity by another. This will give us two components, the numeric value and the compound unit. The numeric component can often be further simplified just as we simplify fractions.
Converting rates allows us to compare rates given in different units and to also obtain a rate in units suitable for a particular application. Common applications are comparing unit prices to find the best deal and comparing speeds of different objects.
Worked example
Example 1
Holly runs a $42$42 kilometre marathon in $3$3 hours and $30$30 minutes.
(a) Find Holly's simplified running rate (speed) in kilometres per hour.
Think: To obtain a rate in km/h, we need to divide the distance in kilometres by the time in hours. ($3$3 hours and $30$30 minutes$=3.5$=3.5 hours)
Do:
| $\text{Speed}$Speed | $=$= | $\frac{42\text{ km}}{3.5\text{ h}}$42 km3.5 h |
Divide distance in kilometres by time in hours |
| $=$= | $12\text{ km/h}$12 km/h |
Simplify the fraction and don't forget units |
(b) Convert her speed to metres per second.
Think: Both the distance unit and time unit are being converted. Let's first change the distance unit. In this case, we are converting distance from kilometres to metres. The number of metres travelled in a given amount of time is $1000$1000 times greater than the number of kilometres, so we want to multiply the rate by $1000$1000.
Then we want to convert the new rate in m/h to m/s. There are $60$60 seconds in a minute, and then $60$60 minutes in an hour, so the number of metres travelled in a second will be the rate in m/h divided by $60^2$602.
Do:
| Speed | $=$= | $12$12 km/h |
Write the given rate including units |
| $=$= | $12\times1000$12×1000 m/h |
Convert the kilometres to metres |
|
| $=$= | $12000$12000 m/h |
Simplify the rate |
|
| $=$= | $\frac{12000}{60^2}$12000602 m/s |
Convert the rate from per hours to per second |
|
| $=$= | $3\frac{1}{3}$313 m/s |
Simplify the rate |
Sometimes a rate can be measured in several different units. For example, speed could be measured in km/h, m/s, mm/s and the units we use will depend on what is reasonable for the speed we are measuring.
A car travelling down the freeway is going to cover many kilometres each hour, so the reasonable units to use would be km/h.
A $100$100-metre sprinter will cover many metres over a smaller period of time, so m/s is a more reasonable unit to describe the speed. (Usain Bolt's $100$100 m world record was achieved at a speed of $12.2$12.2 m/s)
A snail travels a very small distance over a small period of time, so mm/s is a reasonable unit to use. (Speed of a common snail is $1$1 mm/s).
But if we want to compare the speed of a car travelling $40$40 km/h and a sprinter who runs at $12.2$12.2 m/s, we would need to convert one of these rates to the units of the other.
Converting rates allows us to compare rates given in different units and to also obtain a rate in unit suitable for a particular application.
To convert a rate, we want to multiply or divide the rate by the appropriate constant. So to convert a rate in m/min to m/s, we want to divide the rate by $60$60. This is because a rate given in m/min, tells us the number of metres per minute, which is the number of metres per $60$60 seconds.
Worked examples
Example 2
Convert the rate of $20$20 L/h to a rate in L/min.
Think: We first need to identify which unit we are changing. Is it the first unit, the second unit or are we converting both? In this case, we are only changing time from hours to minutes. We also need to identify what the conversion factor between these units is. In our case, $1$1 hour is equivalent to $60$60 minutes.
Do: The number of litres in an hour must be $60$60 times more than the number of litres in a minute. So we want to divide the given rate by $60$60.
| Rate | $=$= | $20$20 L/h |
Write the rate as a division including units. |
| $=$= | $\frac{20}{60}$2060 L/min |
Convert hours to minutes by dividing by $60$60. |
|
| $=$= | $\frac{1}{3}$13 L/min |
Simplify the fraction if possible. |
Example 3
How fast is Usain Bolt's world record speed of $12.2$12.2 m/s in km/h?
Think: Both the distance unit and time unit are being converted. Let's first change the distance unit. In this case, we are changing distance from metres to kilometres. The number of metres travelled in a given second is $1000$1000 times greater than the number of kilometres, so we want to divide the rate by $1000$1000.
Then we want to convert the new rate in km/s to km/h. There are $60$60 seconds in a minute, and then $60$60 minutes in an hour, so the number of kilometres travelled in an hour will be the rate in km/s multiplied by $60^2$602.
Do:
| Rate | $=$= | $12.2$12.2 m/s |
Write the rate as a fraction including units. |
| $=$= | $12.2/1000$12.2/1000 km/s |
Convert metres to kilometres by dividing by $1000$1000. |
|
| $=$= | $0.0122$0.0122 km/s |
Simplify the rate. |
|
| $=$= | $0.0122\times60^2$0.0122×602 km/h |
Convert seconds to hours by multiplying by $60^2$602 |
|
| $=$= | $43.92$43.92 km/h |
Simplify the rate. |
Reflect: Alternatively we can convert km/s to km/h by converting in stages. First we can find the rate in km/m, by multiplying by $60$60 and then we can find the rate in km/h by multiplying by $60$60 again.
When converting units think carefully about if your answer should get smaller or larger and, if you need to divide or multiply by the conversion factor. For instance, the distance you travel in metres in an hour, should be greater than the number of kilometres travelled in an hour. So a rate in m/h should be greater if it was given in km/h.
$\text{1 metre}=\text{100 centimetres}$1 metre=100 centimetres
$\text{1 metre}=\text{1000 millimetres}$1 metre=1000 millimetres
$\text{1 kilometre}=\text{1000 metres}$1 kilometre=1000 metres
$\text{1 litre}=\text{1000 millilitres}$1 litre=1000 millilitres
$\text{1 hour}=\text{60 minutes}$1 hour=60 minutes
$\text{1 minute}=\text{60 seconds}$1 minute=60 seconds
Practice questions
Question 1
Convert $468$468 km/hr into m/s.
First convert $468$468 km/hr into m/hr.
Now convert $468000$468000 m/hr into m/s.
Question 2
Patricia eats $7.2$7.2 litres of ice cream in $6$6 minutes in an ice-cream eating contest. Patricia wants to find her rate of ice-cream consumption in millilitres per second.
Which two of the following unit conversions should Patricia make?
Select both correct answers.
Convert minutes to seconds by multiplying by $60$60
AConvert litres to millilitres by dividing by $1000$1000
BConvert litres to millilitres by multiplying by $1000$1000
CConvert minutes to seconds by dividing by $60$60
DHow many millilitres of ice-cream did Patricia consume?
How many seconds did it take for Patricia to consume all the ice-cream?
What is her rate of consumption of ice-cream in mL/s?