Answer
$2.3\times 10^{12}~~$ alpha particles strike the surface in 3.0 s
Work Step by Step
We can find the total charge in 3.0 seconds:
$q = i~t$
$q = (0.25\times 10^{-6}~A)(3.0~s)$
$q = 0.75\times 10^{-6}~C$
We can find the number of alpha particles:
$\frac{q}{2e} = \frac{0.75\times 10^{-6}~C}{(2)(1.6\times 10^{-19}~C)} = 2.3\times 10^{12}$
$2.3\times 10^{12}~~$ alpha particles strike the surface in 3.0 s
