A roughly spherical volume of rock 2.1 km in radius is 30% denser than the surrounding rock, whose density is 2900 kg/m3. The denser rock is centered 2.5 km below Earth's surface. By what percentage is the surface value of g directly above the denser rock increased due to its excess density? Hint: Treat the "extra" mass of the denser rock as a gravitating sphere, and calculate the gravitational acceleration it produces at Earth's surface.