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 

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