A ball is thrown vertically upwards with a velocity of $19.6 \mathrm{~ms}^{-1}$ from the top of a tower. The ball strikes the ground after 6 s . The height from the ground up to which the ball can rise will be $\left(\frac{\mathrm{k}}{5}\right) \mathrm{m}$. the value of k is $\ldots \ldots\left(\right.$ use $\left.\mathrm{g}=9.8 \mathrm{~m} / \mathrm{s}^2\right)$