Problem 4.13

A point $A$ oscillates according to a certain harmonic law in the reference frame $K^{\prime}$ which in its turn performs harmonic oscillations relative to the reference frame $K .$ Both oscillations occur along the same direction. When the $K^{\prime}$ frame oscillates at the frequency 20 or $24 \mathrm{~Hz},$ the beat frequency of the point $A$ in the $K$ frame turns out to be equal to $v$. At what frequency of oscillation of the frame $K^{\prime}$ will the beat frequency of the point $A$ become equal to $2 v ?$