A tank holds x gallons of a saltwater solution that is 20% salt by volume. One Fourth of the water is evaporated, leaving all of the salt. When 6 Gallons of water and 12 gallons of salt are added, the resulting mixture is 33 1/3 % salt by volume. What is the value of x? Pick one.
A) 37.5  B) 75  C) 100  D) 150  E) 90
Nope, 150. I can only get it by following PR's backsolving explanation. I hate that.   Original mixture has 20% salt and 80% water.   Total = x   Out of which Salt = 0.2x and water = 0.8x   Now, 1/4 water evaporates and all salt remains.   So what remains is 0.2x salt and 0.6x water.   Now 12 gallons salt is added and 6 gallons of water is added.   So salt now becomes -&gt; (0.2x + 12) and water --&gt; (0.6x+6)   Amount of salt is 33.33% of total. So amount of water is 66.66%. So salt is half of the volume of water.   So (0.2x+12) = (0.6x+6)/2 =&gt; 0.4x+24=0.6x+6 -&gt;0.2x=18   Solving, x = 90   Answer : 90
<result>E</result>
A tank holds x gallons of a saltwater solution that is 20% salt by volume. One Fourth of the water is evaporated, leaving all of the salt. When 6 Gallons of water and 12 gallons of salt are added, the resulting mixture is 33 1/3 % salt by volume. What is the value of x?