Question

ill decides to move her fish from a 20 gallon aquarium to a 50 gallon aquarium. She added 3 tablespoons of salt to her old aquarium. If she wishes to maintain to same salinity level, how many tablespoons should she add to her new aquarium?

Answer 1

In the old aquarium, 3 tablespoons of salt were used for 20 gallons. That's a conversion ratio of

$\dfrac{3\,\rm tbsp}{20\,\rm{gal}}=0.15\,\dfrac{\rm tbsp}{\rm gal}$

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

$(50\,\rm{gal})\left(0.15\,\dfrac{\rm tbsp}{\rm gal}\right)=7.5\,\rm{tbsp}$

to maintain the salinity.