HW is correct. But for the recipe that you have, a 10% salt solution would be that 10% of the total solution is salt and the other 90% is water (which you know). But you can't do this in the way of gallons b/c a solid (salt) can't be properly measured in liquid (water) measurements. I, personally, would do this in the matter of weight. just figure that one gallon of water weighs approximately 8 pounds. So, lets just say you want five gallons of this salt quench. 5 gallons=40lbs. (roughly). and 10% salt would be 4lbs. Now, in true mathematical terms, this will not yield you 5 gallons. It would be more b/c you are adding to the already 5 gallons. But since the salt dissolves into the water, the volume will be the same (at least 99% the same) but the density will change. Now, the 5 gallons of water weighs approximately 44 pounds.
I know its a little too drawn out, but I hope this helps.
-Tony