A solution is obtained by mixing 300 g of 25% solution and 400 g of 40% solution by mass.

Total amount of solute present in the mixture is given by,

$300 \times \frac{25}{100}+400 \times \frac{40}{100}$

= 75 + 160

= 235 g

Total amount of solution = 300 + 400 = 700 g

Therefore, mass percentage $(w / w)$ of the solute in the resulting solution, $=\frac{235}{700} \times 100 \%$

= 33.57%

And, mass percentage (w/w) of the solvent in the resulting solution,

= (100 − 33.57)%

= 66.43%