When performing division, the final answer should have the same number of significant figures as the number with the smallest number of sig figs. In this case, both of the given numbers have 3 sig figs, meaning the final answer should have this many as well.
Hot-air balloons float because the air caught inside the balloon is heated up by a burner, making it less dense than the air outside. As the burner heats the air, it expands and some of the air escapes; that's what makes it less dense
