Which two species are most closely related

The volume of helium in a blimp is 6.28 x 10^9 millimeters. the density of helium in the blimp is .1786 kilogram/meter^3. find the mass of the helium in the blimp.

Astudent reacts 13 moles of iron with 21 moles of oxygen according to the following equation:

Athe top of a hill, an athlete on a skateboard has x joules of mechanical energy. how much mechanical energy will she have at the bottom of the hill? ignore the effects of friction.