INTERACTIVE EXAMPLE General Gas Law

Helium$-$filled balloons are used to carry scientific instruments high into the atmosphere. Suppose a balloon is launched when the temperature is $21\mathrm{.}5C,$ and the barometric pressure is $758mmHg.$ If the balloon's volume is $4\mathrm{.}22{10}^{4}L($and no helium escapes from the balloon$),$ what will the volume be at a height of $20$ miles, where the pressure is $76\mathrm{.}0mmHg,$ and the temperature is $-33\mathrm{.}0C?$