Consider the system shown in figure. The pulley and the string are light and all the surface are frictionless. The tension is the string is (Take g = 10m/s2)

Since the string is inextensible , magnitudes of acceleration of the blocks are the same.From  fig (1) , T = 1 X a ....................(1)From fig - (2) , 10 -T = 1 X a ....................(2)Solving , T = 5 N