Let t = tetras and g = guppies
t + g = 48 tetras plus guppies equals 48 total fish
t = 11g 11 tetras for every guppie (It's easy to mix up this equation.)
(11g)+ g = 48 Substituted 11g for t to reduce to only one variable.
12g = 48 Added like terms.
g = 4 guppies Divided both sides by 12
If t = 11g, then there are t = 11(4) = 44 tetras
Do these answers make sense when put back into the question? Yes.