If the average (arithmetic mean) of 2r and 2r-5 is x and if the average of 2r and 2r+9 is y, what is the average of x and y?
(2r + 2r-5) / 2 = x
(2r + 2r+9) / 2 = y
x + y = (8r +4) / 2 = 4r+2
the average of x+y = (4r+2) / 2 = 2r+1
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