A theory question about gasses and pressure

[Zeynaz]

Homework Statement

A few times during summer in Sicily, a giant balloon is released into the highest layers of the atmosphere, the stratosphere. Before release a certain amount of helium is pumped into the balloon. The balloon looks like a gigantic, not well-filled plastic bag. The helium comes from cylinders, each having a volume of 75dm^3. The initial pressure in these cylinders is 2.1e7 Pa and temperature is 25 C.
After the balloon is released, it rises to a height of 38km. Due to the low air pressure at this height , the balloon is fully swollen with a volume of 8.0e5 m^3. The pressure inside the balloon has decresed to 500 Pa at an ambient temperature of -43 C.

---Explain that the pressure in the balloon has decreased, even though it is now fully swollen.

Relevant Equations

PV=nRT

I am struggling to answer questions about gasses. In this case the only way i can think of is, it has to do with the change in atmosphere right? because if we think about the formula P/T=P/T as the temperature decreases the pressure should increase. But in this case we have a smaller pressure.

Could someone explain?
Also, i am struggling to solve these kinds of questions. Do you have any tips i can use or some basic knowledge that can help me to differentiate some ideas.

thanks!

 

[Chestermiller]

Your mathematics is faulty. According to the ideal gas law, if the temperature decreases, the pressure should decrease.