Answer: Choice C
the speed of the car
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Reason:
The term "gradient" is the same as "slope".
Recall that slope = rise/run = (change in y)/(change in x).
The change in y represents how far you travel, as the distance is along the vertical axis. The change in x is the time duration.
Then,
speed = distance/time
which helps show the slope of this distance-time graph is the speed of the car. The steeper the slope, the faster the car.
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An example:
The equation y = 50x has slope 50. The x is the time and y is the distance traveled.
At time x = 0 hours, you have traveled y = 0 miles.
At time x = 1 hour, you have traveled y = 50 miles.
The two points (0,0) and (1,50) are on this line. Use the slope formula to get a slope of 50, which represents the speed of the car in miles per hour (mph). Or note that y = 50x is in the form y = mx+b
m = slope = 50
b = y intercept = 0