A hydrogen balloon at the Earth's surface has a volume?

A hydrogen balloon at the Earth's surface has a volume of 6.0 m3 on a day when the temperature is 24� C and the pressure is 1.00e5 N/m2. The balloon rises and expands as the pressure drops. What would the volume of the same number of moles of hydrogen be at an altitude of 40 km where the pressure is 4.20e2 N/m2 and the temperature is -17� C?