Having a tough time with thermodynamics. This one should be easy, but I got the question wrong. So here goes......

Say we have an ideal gas in a cylinder. The pressure stays constant at 1.3 atm and temperature remains at 200K. The volume is increased from 15L to 50L. 1500J of heat is added to the system during the volume change, yet the temperature remains at 200K and pressure remains at 1.3 atm. Now, how much work is done by the gas?

I figured W = P * change in Volume. That answer was wrong. It did not indicate the correct answer, but said the since heat was added, first the change in internal energy is calculated, then subtract that from 1500J of heat added.

Is this the correct way of calculating this? I'm missing something here.