Two block $\mathrm{A} \mathrm{and} \mathrm{B}$ having masses $m_1=1 \mathrm{kg},m_2=4 \mathrm{kg}$ are arranged as shown in the figure. The pulleys $\mathrm{P} \mathrm{and} \mathrm{Q}$ are light and frictionless. All the blocks are resting on a horizontal floor and the pulleys are held such that strings remains just taut . At moment $t=0$ a force $\mathrm{F}=30 \mathrm{t}\left(\mathrm{N}\right)$ starts acting on the pulley $\mathrm{P}$ along vertically upward direction as shown in the figure. The time when the blocks $\mathrm{A} \mathrm{and} \mathrm{B}$ loose contact with ground is $4/x\mathit{ }\sec$ then $\mathrm{x}$ is