In the old aquarium, 3 tablespoons of salt were used for 20 gallons. That's a conversion ratio of
[tex]\dfrac{3\,\rm tbsp}{20\,\rm{gal}}=0.15\,\dfrac{\rm tbsp}{\rm gal}[/tex]
That is, for every gallon of water in the aquarium, there was 0.15 tbsp of salt. So a 50 gallon aquarium, Ill should add
[tex](50\,\rm{gal})\left(0.15\,\dfrac{\rm tbsp}{\rm gal}\right)=7.5\,\rm{tbsp}[/tex]
to maintain the salinity.
