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The Meaning of Slope and y-Intercept
     in the Context of Word Problems

We have seen that slope measures how much y changes for every so much that x changes. For instance, in the line y = ( ³/₅ )x – 2, the slope is m =  ³/₅. This means that, starting at any point on this line, you can get to another point on the line by going up 3 units and then going to the right 5 units. But we could also view this slope as a fraction over 1:

In other words, for every one unit that x moves over to the right, y goes up by three-fifths of a unit. While this doesn't necessarily graph as nicely as "three up and five over", it can be a more useful way of viewing things when you're doing word problems.

Often, these word problems deal with changes over the course of time. That is, the equations will deal with how much something changes over the course of time. For instance, a problem might deal with how the population grows in a certain city, with the population increasing by a certain fixed amount every year.

When x = 0, the corresponding y-value is the y-intercept. In the particular context of word problems, though, the y-intercept (that is, when x = 0) also refers to the starting value. In the example from above, the y-intercept would be the population when the sociologists started keeping track of the population. Let's do a few examples, to see how this works:

• The average lifespan of American women has been tracked, and the model for the data is y = 0.2t + 73, where t = 0 corresponds to 1960.  Explain the meaning of the slope and y-intercept.

What is the slope? It is m = 0.2.

What is the meaning of the slope? It means that, every year, the average lifespan of American women increased by 0.2 years.

When t = 0, what is the value of y? We get y = 73.

What is the meaning of this y-value? It means that, in 1960 (when they started counting), the average lifespan was 73 years. Copyright © Elizabeth Stapel 2006-2008 All Rights Reserved

• The equation for the speed (not height) of a ball that is thrown straight up in the air is given by v = 128 – 32t, where v is the velocity (in feet per second) and t is the number of seconds after the ball is thrown. With what initial velocity was the ball thrown? What is the meaning of the slope?

What is the slope? It is m = –32.

What is the meaning of the slope? It means that, every second, the speed decreases by 32 feet per second.  Eventually the velocity becomes zero (when the ball reaches its peak), and then becomes negative (when gravity takes over and pulls the ball back down to the ground).

When t = 0, what is the value of y? We get y = 128.

What is the meaning of this y-value? It means that, when the ball was released at t = 0 seconds (when they started counting), it was launched upward at 128 feet per second.

• Fisherman in the Finger Lakes Region have been recording the dead fish they encounter while fishing in the region. The Department of Environmental Conservation monitors the pollution index for the Finger Lakes Region. The model for the number of fish deaths "y" for a given pollution index "x" is y = 9.607x + 111.958. What is the meaning of the slope? What is the meaning of the y-intercept?

What is the slope? It is m = 9.607.

What is the meaning of the slope? In means that, for every increase in the pollution index by one unit (say, from a pollution index of 6 to a pollution index of 7), there are nine or ten more fish deaths during the year.

When x = 0, what is the value of y? We get y = 111.958.

What is the meaning of this y-value? It means that, even if the index is zero (that is, even if the water is utterly pure), there would still be about 112 fish deaths a year anyway.

Word problems with linear (straight-line) equations almost always work this way: the slope is the rate of change, and the y-intercept is the starting value.

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