900mL of 35% sulphuric acid is added to 42% sulphuric acid to create 39% sulphuric acid? How much 42% sulphuric acid was used? (show complete linear systems solutions-decimals may be used)

Answer asap and if you do you will get brainliest. catherine buys a gallon of ice cream from the store. after taking it home, she eats a fifth of a gallon of ice cream. her sister eats some of the ice cream as well. if two-thirds of the original amount of ice cream is left, then what fraction of a gallon of ice cream did her sister eat?