A block of mass m moves with a speed v towards the right block which is in equilibrium with a spring attached to rigid wall. If the surface is frictionless and collisions are lastic, the frequency of collisions between the masses will be :

Time taken to collide on left wall and get back to the mass attached with spring is $t_1=\frac{2L}{v}$. Time to get the spring compressed once and to come back is, $t_2=\frac{T}{2}=\frac{2\pi }{2}\sqrt{\frac{m}{K}}=\pi \sqrt{\frac{m}{K}}$.

$\therefore$ Average time between two successive collisions,  $t=\frac{t_1+t_2}{2}$ collision frequency $=\frac{1}{t}$