Notice that after oven drying, % of Water becomes 0.50 %.
Wt. of dried H20 = 5 % - 0.5 % = 4.5 %
In our original sample, we have 24.0 % P205 and 5 % Water and the rest is unknown, so assume that a 100 % sample is equivalent to 100 grams.
Wt. of the original sample after oven drying = 100 g - 4.5 g = 95.5 grams
% P2O5 remaining after oven drying = (24.0 g/95.5 g) x 100% = 25.13 %