Answer
83 people ordered vanilla.
When flipping one side of the proportion the result, 750 people ordered vanilla, is unreasonable because it exceeds the total number of people.
Work Step by Step
Correct
$\frac{1\ vanilla}{3\ people} = \frac{x\ vanilla}{250\ people}$
$3x = 1\times250$
$\frac{3x}{3} = \frac{250}{3}$
$x = 83$
Incorrect
$\frac{1}{3} = \frac{250}{x}$
$1x = 3\times250$
$x = 750$
