Once you set Cn at 2.5 for working out Va the answer for Va is Vs1g times sq root 2.5.
If you reduce aircraft mass you get a new lower Vs1g and therefore a new
lower Va.
Because the lift equation has Vsq in it the
reduction in Vs1g is proportional to sq root of the change in mass
It is a quirk of the mathematics that, for small changes, inside 20%,
the sq root of the proportional change is approximately half the original
figure, so a 20% reduction in mass gives a 10% reduction in Va.
The answer gives figures of 19/10, which is probably the correct answer,
not just the approximation.