You can compute both the mean and second moment directly using the density function; in this case, it's
Then the mean (first moment) is
and the second moment is
The second moment is useful in finding the variance, which is given by
You get the standard deviation by taking the square root of the variance, and so
Answer: Draw a line through the two points (0,2) and (3,3). See the attached image below.
How did I get these two points? The first point comes from the fact that "at the start, the height is 2 inches" meaning that x = 0 hours and y = 2 inches pair up together. This is show as point A. This is the y intercept as this is the location where the blue graph crosses the vertical y axis.
Point B is found by moving 1 unit up from point A and 3 units to the right to land on (3,3). So we have x = 3 hours and y = 3 inches pair up together.
Note: the slope is rise/run = 1/3 helping us determine the unit rate or speed in which the water is rising (ie the water is rising one-third of an inch per hour)
