Question 110304
f(x)=.8-.1x, where x is the amount drained, so f is the amount of an  anti-freeze stil in the car. We're assuming that this mixtured is "well mixed" of course. 

The amount of fluid drained is equal to the amount of antifreeze we are adding. So .8-.1x+x= the amount of antifreeze in the car after refilling it. We want this number, divided by 8 to be .25. So. {{{.8+.9x=8(.25)), which is 1.333...