Given a 3 box model of the atmosphere,determine fraction of mass transport from lower layer
My results:

If an $\alpha$ sized chunk was moved up, then what remains is $(1-\alpha)$.  Because each layer is 300mb thick, they have equal parts mass.  What goes up must be compensated, and due to conservation of mass, there is no gain or loss of mass in my system.  Potential temperature is a conserved quantity, so I can say:

$$350K(1-\alpha)+325K\alpha = 325K(1-\alpha)+375K\alpha$$
$$\alpha = \frac{1}{3}$$

Further,
$\frac{1}{3}$ mass movements equates to $\frac{1}{3}$ of a $300mb$ layer, which is $100mb$ subsidence.