Question 68408
ALSO SEE PROBLEM #66436 FOR A SIMILAR PROBLEM

let x=amount of 25% solution that needs to be drained and replaced with pure antifreeze 
Then 25-x is the amount of 25% antifreeze left after the system was drained 
Now we know that the amount of pure antifreeze left after the system was drained .25(12-x),plus the amount of pure antifreeze added back in (x) equals the amount of pure antifreeze in the final solution (12)(.50). So our equation to solve is: 

.25(12-x)+x=(12)(.50) simplifying, we get: 
3-.25x+x=6 subtract 3 from both sides: 
.75x=3
x=4 liters needs to be drained and replaced by pure antifreeze----ans 
ck 
.25(12-4)+4=6
2+4=6
6=6

Hope this helps ----ptaylor