Commercial kerosene is stocked in a bulk tank at the beginning of each week. Because of limited supplies, the proportion X of the capacity of the tank available for sale and the proportion Y of the capacity of the tank actually sold during the week are continuous random variables. Their joint distribution is given by

Find the covariance of X and Y.

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S4x if 0 < y s x; 0 sxs1 f(x, y) = elsewhere