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## Get ready for Algebra 1

### Course: Get ready for Algebra 1>Unit 4

Lesson 4: Linear models

# 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
• 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 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
• 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.
• Where does the 2.5 come from? No where in the original question does it tell us how much she is spending at a time. Explain the 2.5 data, please.
Dana Goodale
• She spends 2 1/2 dollars a day or 2.5x where x is the number of days.
• 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.
• Agent Hunt transferred classified files from the CIA mainframe onto his flash drive. The drive had some files on it before the transfer, and the transfer happened at a rate of 4.4 megabytes per second. After 32 seconds, there were 384 megabytes on the drive. The drive had a maximum capacity of 1000 megabytes.
How full was the drive when the transfer began?

How long from the time that Agent Hunt started the transfer did it take the drive to be completely full?

is he a hacker?