A hanging mass $M$ is connected to a four times bigger mass by using a string-pulley arrangement. as shown in the figure. The bigger mass is placed on a horizontal ice-slab and being pulled by 2 Mg force. In this situation. tension in the string is $\frac{x}{5} M g$ for $x=$ $\_\_\_\_$ Neglect mass of the string and friction of the block (bigger mass) with ice slab. (Given $\mathrm{g}=$ acceleration due to gravity)