#### Answer

52 tetras

#### Work Step by Step

Write a proportion. Size is to size as count is to count.
$\frac{20}{55}=\frac{19}{x}$ $\longrightarrow$cross multiply to solve for x
$20x=55\times19$ $\longrightarrow$multiply
$20x=1045$ $\longrightarrow$multiplication property of equality
$20x\div20=1045\div20$ $\longrightarrow$divide
$x=52.25$
Pete would not put a part of a tetra in the tank so the answer is 52.