Let x be the number of gallons of fresh water (no salt) and y be the number of gallons of 30% salt water.
This means that all the salt in the final solution comes from the original 30% solution. So, let's find the total amount of salt in the final solution:
Salt content in final = 90 * .23 = 20.7 gallons
Now, since all salt comes from the 30% solution, we can find the number of gallons of 30% this way:
.3y = 20.7 (since 30% of the original solution is salt)
y = 69 gallons in the original 30% solution.
We also know that the sum of the gallons of fresh water and 30% solution is 90.
x + y = 90
x + 69 = 90
x = 21 gallons of fresh water need to be added.