Step 1
Given; Every 5th person who enters the cafeteria is asked what their favorite food that is being served is.
Required; To determine if it is biased or unbiased
Step 2
In a biased sample, one or more parts of the population are favored over others, whereas in an unbiased sample, each member of the population has an equal chance of being selected.
Since every 5th person who enters the cafeteria is asked what their favorite food that is being served is, then the 5th person could be anybody and each member of the population has an equal chance of being selected. Anybody could be the 5th person on different days hence the situation is unbiased.
If one or more parts of the population were favored then it would have been biased but everyone has an equal chance of being the 5th student.
Therefore the situation is systematic but it is unbiased