Math Equation? Hypo

[stdreb27]

So I'm fixing to hypo QT tank for the first time. I was wondering if there was a math equation to figure out how to lower the salinity equally?
You know add a gallon of fresh water, the salinity in a x sized tank will drop y amount.

[stevensun]

Assume your salinity in a 50g tank is 1.025 and you add X gallons of fresh water with salinity of 1. The salinity you get can be calculated as:

[(1.025)(50-X) + (1)(X)]/50