Let the acceleration of each mass be a and tension on the string be T. If g represents the acceleration due to gravity then,
For the motion of mass of 10kg we can write
5g-T=5a .........(1)
For the mass of 1kg we get
T- g=a............(2)
Adding (1) and (2)
6a= 4g
=> a = 2/3 m/s^2
So T = g+2/3g=5/3g
So reading of the spring balance will be
=(2T)/g=(2x(5/3)g)=10/3kg