Newtonian mechanics (elevator problem)

The question is to find the reading of the spring balance. I want to solve it without using pseudo forces. What should be the method to go about using relative acceleration from ground frame? It would be helpful if someone could help me with the equations from ground frame

So as you don't want to use the pseudo force concept we can without loss of generality assume acceleration of the blocks with respect to the pulley as $$a$$ and also assume that in the frame of the elevator the $$3.0 kg$$ block is going down then the acceleration with respect to the ground frame of both the blocks can be set up as $$a_{3kg} =a-\frac {g}{10}$$ and $$a_{1.5kg}=a+ \frac {g}{10}$$ and then you could use the Newton's second law to solve for $$a$$ and the tension in the string connecting the blocks $$T$$. The reading of the balance would be $$2T$$ if the pulley is massless.