Answer:
7.5 tablespoons of salt
Step-by-step explanation:
The old aquarium had 20 gallons of water while the new aquarium had 50 gallons of water.
If 20 gallons of water required 3 tablespoons of salt
50 gallons of water will require: 50 x 3/20 = 7.5 tablespoons of salt
Hence, 7.5 tablespoons of salt should be added to her new aquarium.