Mass percent of a solution can be defined as grams of solute per 100 grams of solution. In this problem, we have 50.0 g of solute and 150.0 g of water. The total mass of the solution will thus be 50.0 g + 150.0 g = 200.0 g. The mass percent will be 50.0g/200.0 g (x100%) = 25.0 %. You can also look at it another way. Since you have 50 g solute in 200 g solution, you will have 25 g solute in 100 g solution. By the definition provided above, this is a 25% by mass solution.