A flag is raised while an onlooker watches from a distance of 20 feet away from the base of the flag pole (see the figure below). The flag rises vertically at a rate of 2 inches per second. Let t denote the time (in seconds) after the flag begins to rise (For simplicity, assume that when the flag begins to rise it is 0 inches above the ground). Express the distance d (in feet) between the flag and the onlooker as a function of t.

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