Two blocks of masses 10 kg and 20 kg are connected by a massless spring and are placed on a smooth horizontal surface. A force of 200 N is applied on 20 kg mass as shown in the diagram. At the instant, the acceleration of 10 kg mass is $12{\mathrm{ms}}^{-2}$, the acceleration of 20 kg mass is  (in $\mathrm{m}/{\sec }^2$).