Question 1001604
If he drains x gallons, then
.25(4-x) + 1.00x = .40(4)
1 - .25x + x = 1.6
.75x = .6
x = .8 
So, after draining .8 gals, he has 3.2 gals of 25% antifreeze. This means he has That means he has 3.2(0.25)= .8 gals of antifreeze in the radiator. 
Now he adds .8 gals of pure antifreeze. Then he has 1.6 gals of antifreeze in 4 gals of mixture. That's .16/4 = 40% antifreeze. We've got the right answer.