Percentage composition in terms of mass of a solution obtained by mixing 300g 25% solution of ${\mathrm{NH}}_4{\mathrm{NO}}_3$ with 400g of a 40% solution of solute 'X'.

$\left({\mathrm{NH}}_4{\mathrm{NO}}_3\right)$

Mass of components $=\frac{2.5}{100}\times 300$

=75g

Total mass of solution = 300 + 400 = 700 gram

Total mass of components = 75 + 160

(Solute ) = 235 gram

Mass % of solute $=\frac{235}{700}\times 100$

= 33.5 %

Mass % of solvent = 100 − 33.5

=66.5%