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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