In total she wants to get 4 gallons of punch
Three gallons of soda are needed for such a mixture, that means she as to add one gallon of fruit on the 3 gallons of soda to get 4 gallons of punch
That is 3/4 (soda) + 1/4 (Juice) = 0.75 Soda + 0.25 Juice
Hence she needs 0.25 of Juice or 25% of juice