In the Coolidge tube experiment, if the applied voltage is increased to three times, the short wavelength limit of continuous X-ray spectrum shifts by$20 \mathrm{pm}$. What is the initial voltage (in $\mathrm{kV}$) applied to the tube to the nearest integer value? [Take $hc = 12400 \mathrm{eV} \stackrel{\circ }{\mathrm{A}}$, $e = 1.6 \times {10}^{-19} \mathrm{C}$]