If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

# Linear function example: spending money

Sal solves an interesting application problem using a linear model. Created by Sal Khan and Monterey Institute for Technology and Education.

## Want to join the conversation?

• When Sal makes the graph, doesn't the x and y-axis have to have the same increments?
• they don't have to have the same increments but it's just ussually that they do
• Why did he make a whole line of answers when the question clearly states that x=8?
• Well, I guess he wanted to show us how to do this.
Also for a clear graph
• but taking the graph takes much time than substituting the value of "x" in the equation.so,what is the use of solving with graph?
• Sal started to hint at the importance of graphs with this word problem by assigning different types of values to each axis (money and days). Graphs, or charts, are used a lot in research and business to help visualize data. As you move into other math topics, including geometry and trig, you will start to see more practical uses for graphing, but you have to start somewhere to gain the fundamentals those other topics and uses rely on.
• why are almost all these comments irrelevant to the video-
• why can't she be in the hole?()
• In this case He is saying that she won't go into debt, the graph goes into
the negative Y quardinate but for this example we are just not looking at those values.
• What if the x in the table is money and the y in the table is the days? I figured out that it is harder to do the equation plugging in the numbers( Y=40 - 2.5x while Y=8 ). Is there anyway I can know which way is easier like sal always does? Does he do the equation in his head before he gives his explanation to find which one is easier?
• Generally speaking, x is our independent variable and y is our dependent variable. That is, y is the variable that is determined by the other variable. If you did y=40-2.5x when y=8, you are solving a different problem. You are finding out how many days pass before she has \$8 left.
• helloooo:) have a great day even though you have to do life
• Excellent video, but what is the reason for not scaling the coordinates proportionately? (So that the slope can accurately represent the relation).
• Suppose that the relation weren't a few tens of dollars per day but millions of dollars a day. You'd have to have a very tall piece of paper to have millions of equally spaced tick marks on the y axis for every tick on the x axis. Or suppose that it were \$0.01 every 3000 days, that would be a very wide piece of paper to have scaled at 1:1.

So, the reason for not scaling at 1:1 is to make the graph usable and practical. Thus, you scale to whatever proportion suits your needs.