Answer
$\displaystyle \frac{25}{3+x}$
Work Step by Step
To get the concentration, we divide the amount of salt (25 oz) by the volume of the mixture. The total volume will be the sum of the 3 gallons of brine and $x$ gallons of water: $3+x$
Thus the concentration is:
$\displaystyle \frac{25}{3+x}$
