$f\left(t\right)=500{\left(0.5\right)}^{\frac{t}{12}}$

The function $f$ models the intensity of an X-ray beam, in number of particles in the X-ray beam, $t$ millimeters below the surface of a sample of iron. According to the model, what is the estimated number of particles in the X-ray beam when it is at the surface of the sample of iron?