You can 
put this solution on YOUR website! 
Hi,
Let x represent the amount of pure antifreeze that must be added 
*Note: the pure antifreeze would be an 100% 'solution' of antifreeze
write as we read***
.20*12gal + 1*x = .80(12gal + x)
solve for x
.2x = .60*12gal
x = 36 gal of antifreeze needs to be added
checking our answer 2.4 + 36 = 38.4