Light of wavelength 250 nm and power 2.0 mW falls on potassium, with a work function of 2.30 eV…

Step 1. Photon. $E = \dfrac{1240}{250} = 4.96\ \text{eV}$.

Step 2. Energy. $KE_{\max} = 4.96 - 2.30 = 2.66\ \text{eV}$.

Step 3. Electrons. $\dfrac{2.0 \times 10^{-3}}{4.96 \times 1.602 \times 10^{-19}} = 2.52 \times 10^{15}$ each second.

Step 4. Current. $2.52 \times 10^{15} \times 1.602 \times 10^{-19} = 4.0 \times 10^{-4}\ \text{A}$.