Okay, the question I'm suppose to answer is: If the attenuation coefficent of Al is 1.73/cm for soft-xrays, what percentage of X-rays penetrate a piece of lead 3.00mm in thickness . . . What I seem to have is I = Io*e^(-ux) I believe u = 1.73/cm and x = 3.00mm. My 3 questions are: 1) Am I going in the correct direction? 2) How do I determine what Io be? (soft x-rays?) 3) Can someone point me to the FAQ for the equation editor? Thanks in advanced.