A solid, uniform, spherical boulder (I=2/5 MR^2) starts from rest and rolls down a hill. The top 1/4 of the hill is rough enough to cause the boulder to roll without slipping, but the lower 3/4 is covered with ice and there is no friction. The translational speed of the boulder when it reaches the bottom of the hill is 20.0m/s. How high is the hill?