The graph of a uniformly distributed population is a rectangle with total area of 1.
Its base is 5 units long. The formula for the area of the whole rectangle is
A = (base)(height) and the area = 1 and base = 5, so substituting,
1 = (5)(h)
So it is 1/5 of a unit high.
The shaded rectangle has a base of 2.75. So using the same formula,
A = (base)(height) and the base = 2.75 and height = 1/5, so substituting,
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Edwin