note sure if you have this solved yet, but how about I through in a couple of hints

this is henry's law.

Pa = k * Ca

where

Pa = partial pressure of component "a" over a liquid

k = a constant (provided the temp is constant)

Ca = concentration of component "a" in the liquid phase

you have t conditions of Pa and Ca... so what if we do this?

Pa1 = k * Ca1

Pa2 = k * Ca2

divide

Pa1 / Pa2 = (k / k) * (Ca1 / Ca2)

simplify and rearrange

Ca2 = Ca1 * (Pa2 / Pa1)

now.. that's all in partial pressures. We don't have partial pressures, we have total pressures. But let's make the assumption that

Pa2 / Pa1 = Ptotal 2 / Ptotal 1

which will be close in this case

so that

Ca2 = Ca1 * (Pa2 / Pa1)

you get to finish