300 g of 25% solution contains solute =75g

400 g of 40% solution contains solute = 160g

Total mass of solution = 300 + 400

$\Rightarrow 700$ g

% of solute in the final solution = $\large\frac{235}{700}$$\times 100$

$\Rightarrow 33.5\%$

% of water in the final solution =100-33.5

$\Rightarrow 66.5\%$