A river descends $15 \mathrm{~m}$ through rapids. The speed of the water is $3.2 \mathrm{~m} / \mathrm{s}$ upon entering the rapids and $13 \mathrm{~m} / \mathrm{s}$ upon leaving. What percentage of the gravitational potential energy of the water-Earth system is transferred to kinetic energy during the descent? (Hint: Consider the descent of, say, $10 \mathrm{~kg}$ of water.)