Reactant nuclei and product nuclei

[gxc9800]

Homework Statement

in the photo, the 23Na form 24Na Am i right?

if i am right, the mass of 23Na should decraese , and mass of 24Na should increase.
but why the solution provided is the mass of 24Na decreses and increases at the time of delta t ?? i can't understand

Homework Equations

The Attempt at a Solution

 

[mfb]

23Na can capture a neutron to become 24Na.
The number of 23Na atoms decreases, but at a negligible rate.
The number of 24Na atoms increases by the production and decreases from the decays - if you start from 0 and have a constant production rate the net effect is always an increase (but getting slower and slower close to the equilibrium), and this is represented by the formula.