Asolution was prepared by dissolving 40.0 g of kcl in 225 g of water. calculate the mass percent of kcl in the solution.

15.1%  
This is a simple matter of addition and division. The total mass of the solution is the sum of the mass of KCl and the mass of the water. So: 
40.0 g + 225 g = 265 g  
And the mass percentage of KCl then becomes the mass of KCl divided by the total mass. So: 
40.0 g / 265 g = 0.150943396 = 15.0943396%  
Since we only have 3 significant figures in our data, round the result to 3 figures, giving 15.1%