Let the number of antifreeze gallon is x
Since the solution contains 95% pure antifreeze, then multiply x by 95% after changing it to decimal

Since they need 285 gallons containing 20% pure antifreeze, then multiply 285 by 20%

Equate 0.95x by 57

Divide both sides by 0.95

Subtract 60 from 285

They need 225 gallons of water and 60 gallons of premium antifreeze to mixe them
Water: 225 gallons
Premium antifreeze: 60 gallons