Hello again MHF!

I have a some questions that I need a little help with concerning air density and buoyancy.

(1):

The density of air near the earth's surface is 1.29 kg/m^3. If a helium balloon with a mass of 1kg floats in air with out rising or falling, what is the minimum volume of the helium in the balloon? Show your calculations.(presume that the mass of the material making up the balloon is negligible.)

I admit that I have no idea where to start with this one.....

(2):

A balloon filled with one liter of air is tied to a brick and dropped from a height of 4600m above sea level. If the atmosphere 4600m above sea level has a pressure of 70kPa, what will the volume of the balloon be when it reaches sea level? Show your calculations.

Same story sadly, I have no idea what to do with this one.

Thanks in advance for your help!