Question 861253
A thorough lesson on this type of exercise problem is in this link:


<a href="http://www.algebra.com/tutors/Two-Part-Mixture-with-one-material-quantity-unknown.lesson?content_action=show_dev">two-part mixture for known final concentration and one part known but other part unknown.</a>
http://www.algebra.com/tutors/Two-Part-Mixture-with-one-material-quantity-unknown.lesson?content_action=show_dev



I believe you were or are missing a denominator expression representing the total amount of resulting solution.  What you showed in your equation is a quantity of salt as expression, and a fraction of salt for resulting solution.  


Study the lesson; but also understand that concentration is a ratio like this:
(amount of solute)/(amount of solution containing the solute).
You want this RATIO to be 0.25 if keeping with decimal fractions; or 25 if keeping with percentage concentration units.



Final answer seems to be 6 grams.  This result checks.