Bunuel

A certain harbor has docking stations along its west and south docks, as shown in the figure; any two adjacent docking stations are separated by a uniform distance d. A certain boat left the west dock from docking station #2 and moved in a straight line diagonally until it reached the south dock. If the boat was at one time directly east of docking station #4 and directly north of docking station #7, at which docking station on the south dock did the boat arrive?
A. #7
B. #8
C. #9
D. #10
E. #11
Let’s rethink this problem as one that involves the equation of a line. Station 5 is the origin (0,0), and all other stations take on the traditional values on the x and y axes. Thus, when the boat leaves station #2, the ordered pair is (0,3), and the line of the boat’s path passes through the point (station #7, station #4), or (2,1). The line that connects these two points has a slope of (3 – 1)/0 – 2) = 2/-2 = -1. Using y = -1x + b, we substitute the values from (0,3), obtaining 3 = (-1)(0) + b, and so b = 3. Thus, the equation of the line connecting the two points is y = -x + 3.
We now plug in the ordered pairs of the answer choices to determine which of their equivalent ordered pairs satisfies the equation y = -x + 3.
Choice A: station #7 is at (2,0). Does 0 = -2 + 3? No.
Choice B: station #8 is at (3,0). Does 0 = -3 + 3? Yes!
Answer: B
Thank you for this helpful explanation. How do you know thought that the graph is going up by the same increments on both axes? For instance, what if each dash on the x axis goes up in intervals by 2 and each dash on the y axis goes up by intervals of 3? How do you know that the graph is spread out even in constant increments at all (e.g., what if from point 6 to 7 was 3 and from points 7 to 8 was 4?) I was just confused how to interpret this graph overall given that there is no "key" if you will to understand if things are draw to scale/we can assume each dash represents 1 unit. Thanks again.