Sometimes multiple conversions are needed before we end up with the units we want. For example, How many minutes are there in 2 days? The process is the same, we just repeat the steps for every unit conversion we make.
Steps for Unit Conversions
Look at the units you have.
Figure out the units you want.
Find the conversion factors that will help you step by step get to the units you want.
Arrange conversion factors so that unwanted units cancel out.
The following video will show an example of using two conversion factors.
Time is one of the most commonly used conversions. Depending on what you are converting between, it is also a good example of sometimes needing more than one conversion factor.
Example:
How many minutes are there in two days?
Unless you know the conversion for minutes to days, this takes two conversion factors.
We can use the following equivalence statements to make our conversion factors.
1 day = 24 hours
1 hour = 60 minutes
First, we start with what we have which is two days.
Next, we use one of our equivalence statements to make the conversion factor that will allow us to cancel out “days.”
This leaves us with “hours” in the numerator. We still need another conversion factor to cancel out the “hours.” The conversion factor \(\frac{60\text{ minutes}}{1\text{ hour}}\) allows us to cancel out “hours” in the numerator with “hours” in the denominator.
\(\frac{\text{Everything in numerator}}{\text{Everything in denominator}}=\frac{5000\times1\times1\:\text{day}}{60\times24}=\frac{5000}{1440}= 3.47222\text{...days}\)
So 5000 minutes = 3.5 days (rounded to nearest tenth.)
Zig-Zag Method
The other method to properly calculate several fractions being multiplied together is to use the zig-zag method. The zig-zag method says to calculate the numbers going in a zig-zag pattern starting with the first numerator.
Any time you go down to the denominator you divide.
Any time you go up to the next numerator you multiply.
This method makes putting the numbers into your calculator very quick. In this case, we enter the following into our calculator going from left to right:
A trip from Los Angeles to New York by car is expected to take about four days of driving time (non-stop). How many hours will a person drive if they make this trip? Use the following information to convert this trip to hours: 1 day = 24 hours
How many seconds are there in 2.5 hours? Use the following information to convert this time to seconds: 1 hour = 60 minutes 1 minutes = 60 seconds
Sara trained for a 10-kilometer race for 18 weeks by running one hour every day. How many minutes did she run altogether during her training? Use the following information to convert her running time to minutes: 1 week = 7 days 1 hour = 60 minutes
The running time for a new children’s movie is 6600 seconds. What is the running time for the movie in hours? Use the following information to convert the running time to hours. Round to the nearest hundredth. 1 minute = 60 seconds 1 hour = 60 minutes
Michael Phelps swam the 200-meter individual medley in 1 minute and 54 seconds. How long did it take him to swim this race using seconds only? Use the following information to convert his time to seconds: 1 minutes = 60 seconds
How many hours are there in eight weeks? Use the following information to convert this time to hours: 1 week = 7 days 1 day = 24 hours
I have a 15 pound turkey. The instructions say to cook it for 12 minutes per pound. The timer uses hours. How many hours should I set the timer for? Use the following information to find how many hours to set the timer for: 60 minutes = 1 hour
Step 1: Find the units we have. In the problem we are told that Sara is running 18 weeks for one hour per day, so the units we have are weeks.
Step 2: Figure out what units we want. The problem asks us to convert weeks to minutes, so the units we want are minutes.
Step 3: Find conversion factors that will help get the units we want. We need to convert weeks to days to hours to minutes so we will need the following conversions:
1 week running = 7 days running
1 day running = 1 hour running (If we were solving for the total number of hours in 18 weeks then we would use the conversion 1 day = 24 hours. Since the runner only runs for 1 hour per day, that is the unit conversion we use instead.)
1 hour running = 60 minutes running
Step 4: Arrange conversion factors so unwanted units cancel out. We know that Sara has been training for 18 weeks, so we need to change weeks to days. To do that we multiply \(\frac{18\:\text{weeks}}{1}\) by \(\frac{7\:\text{days}}{1\:\text{week}}\):
Since Sara is training for 1 hour per day, we can convert days to hours by multiplying by \(\frac{1\:\text{hour of running}}{1\:\text{day}}\), so now we have:
The last conversion factor we need to include is multiplying by \(\frac{60\:\text{min}}{1\:{\color{Blue} \text{hr}}}\) to change hours to minutes. Now we have:
Step 1: Find the units we have. In this problem, we have a 15-pound turkey, so the units we have are pounds.
Step 2: Figure out what units we want. The problem asks us to find out how many hours we need to cook the turkey for, so the unit we want is hours.
Step 3: Find conversion factors that will help get the units we want. We are given the conversion factor 12 minutes = 1 pound, and we will also need 60 minutes=1 hour.
Step 4: Arrange conversion factors so unwanted units cancel out. We know that our turkey is 15 lbs, which need to be converted to minutes, then to hours. We can set it up like this: