Question 366644
let x = # gallons pure water that must be added.  Then 
10*0.30 + x*0.00 = (10 + x)0.25,
3  = 2.5 + 0.25x,
0.5 = 0.25x
x = 2 gallons of water must be added.

***Note:  x*0.00  denotes the weight contribution of the water to the salt content of the newly mixed solution. x is in gallons, and its  salt concentration is 0.00 weight units per gallon, so that x*0.00 is in weight units.***