Calculate the mass by mass percentage of the solution obtained by mixing 400 g of a 25% (m/m%) solution and 500 g of a 40% (m/m%) solution.

The correct option is A 33.33%
25% (m/m) means 25 g of a solute is present in 100 g of the solution.
40% (m/m) mass means 40 g of a solute is present in 100 g of the solution.
Mass of the solute in 400 g of the solution= $25\times \frac{400}{100}$ = 100 g
Mass of the solute in 500 g of the solution = $40\times \frac{500}{100}$ = 200 g
Therefore, total mass of the solute = 100 + 200 = 300 g
Total mass of the solution = 400 + 500 = 900 g
Required mass percent composition = $\frac{\text{Total mass of the solute}}{\text{Total mass of the solution}}\times 100$
= $\frac{300}{900}\times 100$ = 33.33%