3 is the beat frequency. A beat frequency is caused by two waves with similar frequencies interfering with each another. The difference in the freq.s is equal to the beat freq. So the second note had to be either 789 or 783 Hz.
first answer is correct: I is proportional to the inverse square since the source emits energy in spherical wave fronts and the surface area of a sphere is 4*pi*r^2.
the second anser sounds good but i haven't a calculator near me. The period of a pendulum is proportional to the square root...