Two masses of $10\; kg$ and $20\; kg$ respectively are connected by a massless spring. A pull of $100 N$ acts on the mass $20 kg$. If the instantaneous acceleration of 10 kg mass is $2 m/s^2$, what is the acceleration of $20 kg$ mass

Question

$$(a)\;1 m/s^2 \quad (b)\;2 m/s^2 \quad (c)\;3 m/s^2 \quad(d)\;4 m/s^2$$