The volume of a cone V =. Since we are given that h = 2r (where 2r is the diameter), V = = = . (1) The rate of the volume change in time is given; it is 15 ; therefore 15 = = = (calculate from the formula (1)) = . = (substitute here h= 5 ft) = . . It implies = = = = 0.76 ft/min. (rounded) Answer. At this moment, the height of the cone is increasing at the rate of 0.76 ft/min. (approximately)