a balloon containing 8 moles he in 0.85 L at 25 degrees

a balloon containing 8 moles he in 0.85 L at 25 degrees

a balloon containing 8 moles he in 0.85 L at 25 degrees celcius is released where it flies to an altitude of 20k feet where the temperature is -35 degrees celcius and the volume expands to 1.66L.  
assume air pressure is constant at 0.95atm.
why do we calculate entropy using delta s = delta s (isothermal expansion)+ delta s (constant volume cooling)?
it seems as if the balloon changes temperature and changes volume. 
why do we use the formulas for constant temperature and constant volume and then sum the results to get the total entropy?

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