Exponential and logarithmic functions

The amount of radioactivity that passes through a barrier is represented by the function Q(x)= 100(0,45)^x/3 where Q is the percentage of radioactivity that passes through the barrier and x is the thickness of the barrier, in centimeters. If 95% of the radioactivity must be blocked, how thick must the barrier be, to the nearest tenth of a centimeter?

Please help!!