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

Mass of component in first solution = 25% of 300 gram  = 25*300/100 = 75 gram

Mass of component in second solution   = 40 % of 400 gram = 40*400/100 = 160 gram

Total mass of solution = 300+400 = 700 gram

Total mass of component  = 75+ 160  = 235 grams

Mass % of final solution     = mass of component * 100 / total mass

                                         =235*100/700  = 33.5%

Total of % is always 100

So % of water in solution = 100 – 33.5  = 66.5 %