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A speed limit sign, something we see every day, provides great insight into the relationship between distance (d), rate (r), and time (t). Take for instance a speed limit of 40 miles per hour. Speed is a rate: in this case, it's the ratio between miles and hours. Miles is a unit of distance, and hours is a unit of time. Therefore, we can say that rate equals distance divided by time.
We can use this relationship to write a few equivalent equations:
In this lesson, we will work primarily with distance = rate • time or .
In some situations, more than one rate has an effect on the situation, which in turn affects how we represent the situation mathematically. For example, imagine swimming across a river or lake. In one direction, the current of the water might work in your favor, making you faster, or at least making it easier for you to keep a certain pace. In the other direction, the current of the water might be working against you, making you slower, or making it harder for you to keep a certain pace.
When two rates work with each other (such as swimming with the current), we can add the two rates together. When rates work against each other (such as swimming against the current), the opposing rate is negative, leading to an expression involving subtraction.
Similar to the mixture problems, when solving a distance, rate, time problem, you need to:
EXAMPLE
A swimmer can swim a half-mile with the current in 6 minutes. However, traveling back against the current, the same distance takes 15 minutes. Assuming the swimmer travels at a constant speed during both trips, what is the speed of the swimmer, and what is the speed of the current?Using our system of equations, add the equations together and the term cancels | |
Divide both sides by 2 | |
Rate of the swimmer |
Using the first equation, substitute the 3.5 for | |
Subtract 3.5 from both sides | |
Subtract 3.5 from both sides |
Source: ADAPTED FROM "BEGINNING AND INTERMEDIATE ALGEBRA" BY TYLER WALLACE, AN OPEN SOURCE TEXTBOOK AVAILABLE AT www.wallace.ccfaculty.org/book/book.html. License: Creative Commons Attribution 3.0 Unported License