chemistry mass relations help.can those of you smart in chemistry help?

[jeez im dope.]

percentage by mass states the number of grams of solute per 100 grams of solution..

what is the concentration in percentage by mass when 18.0 grams of potassium nitrate is dissolved in 150 grams of water?

mayb if this gets answered i can finish the sheet
i have no idea how to do this..
to the person that most clearly explains it, correctly too, gets best answer

[M W]

% concentration by weight = mass solute / mass total x 100%

the solute is the thing dissolved = potassium nitrate
the solvent is the thing that does the dissolving = water
total mass = sum of all components

so mass solute = mass potassium nitrate = 18.0 g

mass total = 18.0 g + 150 g = 168.0 g

so % by weight = 18.0 g / 168.0 g x 100% = 10.7%