Multiple Choice: How many otters are there in a group of 27 penguins and otters if there are four fifths as many penguins as otters?
My reasoning is $\frac{1}{5} \cdot \frac{27}{1}$ are otters. So there are 5.4 otters out of 27. ~6 or (A) 6.
Choices: (A) 6 (B) 15 (C) 21 (D) 22
The answer is stated as: (B) 15
Starting first, as pointed out by @Ajay and @Amaan M, this equation uses a Simultaneous Equation that can be used to solve for a single variable.
$x$ = penguins, $y$ = otters.
This equation is used because both the $x$ and $y$ variables add up to $27$.
Simultaneous Equation: $x + y = 27$
Because x is equal to $\frac{4}{5}$ of y, which is "Four Fifths as many Penguins as otters", we can say $x = \frac{4}{5}y$
Substitution: $y + \frac{4}{5}y = 27$
Evaluation: $\frac{4}{5}y + \frac{5}{5}y = 27 \Rightarrow \frac{9}{5}y \cdot \frac{5}{9} = \frac{27}{1} \cdot {5}{9} \Rightarrow y = \frac{135}{9} \Rightarrow y = 15$