A high-altitude research balloon is inflated on the ground using a portable helium cylinder.
The cylinder has a volume of 6.4 litres6.4\text{ litres}6.4 litres and contains helium compressed to 150150150 times normal atmospheric pressure. The helium is released to inflate the balloon at normal atmospheric pressure. The balloon is inflated at a constant flow rate of 16 litres16\text{ litres}16 litres per minute (measured at normal atmospheric pressure).
Calculate the maximum duration (in minutes) that the cylinder can continuously supply helium at this rate.
Once released, the balloon rises from the ground up through the atmosphere. Assuming the temperature of the helium remains constant, explain why the balloon expands as it rises.