This problem came from Fuel cell fundamentals 3rd edition Wiley

Consider two H2-O2 PEMFCs powering an external load at 1A/cm2 and 70oC. Hydrogen gas at 90oC and 80% relative humidity is provided to the fuel cell at the rate of 8A. The fuel cell area is 8 cm2 and the drag ratio of water molecules to hydrogen, , is 0.8. Fid the water activity of hydrogen exhaust. Assume that p=1atm and that the hydrogen exhaust exits at the fuel cell temperature, 70.