A balloon is filled with 3.00 l of helium at a pressure of 765 torr. what is the volume (l) of the balloon at an altitude where the pressure in the balloon is 70,661 pa?

We assume ideal gas behaviour so we can use the formula:

PV = nRT

From the problem, we can say that the amount of Helium does not change so n is constant and that we assume no change in temperature so that T is also constant, therefore:

P V = constant

And then:

P1 V1 = P2 V2

765 torr * 3.00 L = 70,661 Pa (760 torr / 101325 Pa) * V2

V2 = 4.33 L