modern physics Questions

The minimum wavelength of X-rays produced by an electron accelerated through a potential difference of $V$ volts is proportional to:

The work functions of Caesium (Cs), Potassium (K) and Sodium (Na) are $2.14\text{eV}$, $2.30\text{eV}$ and $2.75\text{eV}$ respectively. If incident electromagnetic radiation has an incident energy of $2.20\text{eV}$, which of these photosensitive surfaces may emit photoelectrons?

The half-life of a radioactive substance is $20\text{minutes}$. In how much time, the activity of the substance drops to ${\frac{1}{16}}^{\text{th}}$ of its initial value?