Finding the volume of this problem while using the Ideal Gas Law

A balloon is partly inflated with 4.80L of helium at sea level where the atmospheric pressure is 988 mbar. The balloon ascends to an altitude of 3000 meters, where the pressure is 847 mbar. What is the volume of the helium in the balloon at the higher altitude? Assume that the temp. of the gas in the balloon does not change in the ascent. 

I think I'm supposed to use the ideal gas law, but I can't figure out what values match with what variable. If someone could show me how to get to the answer that would be great!

P1V1/T1 = P2V2/T2

Let the '1' conditions be the higher altitude and the height does not come into the calculation)

So V1 = (P2V2T1)/(P1T2)

As temperature is constant

V1 = (P2V2)/(P1)