Slope = rate
Here's another graph:
Looking at the graphs, can you figure out how much each person makes per hour? In other words, what the change in money per time is?

You may recognize "change in money per time" as being basically the same as delta y / delta x, which is the classic formula for slope. This is no accident.
If you can engrave this on your brain, you'll have a head start interpreting lots of graphs, whether they are straight lines or not. The slope at any part of a graph tells you how fast the process is happening right then.
This graph represents sales of pizza slices over a period of 4 hours. At what point in time is pizza income increasing at the fastest rate?
(To make this problem interactive, turn on javascript!)
 I need a hint ... : Slope = Rate
 ...another hint ... : Where is the slope the steepest?
I think I have the answer: At about the 1.5 hour mark.
To make this element interactive, turn on javascript.
Click the buttons to see how slope and rate are related:

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