To find out how many hours Mark worked, we can use the formula:
\[ \text{Hours Worked} = \frac{\text{Total Pay}}{\text{Pay per Hour}} \]
Given that Mark's total pay is $115 and he earns $5.75 per hour, we can calculate the hours as follows:
\[ \text{Hours Worked} = \frac{115}{5.75} \]
Now performing the division:
\[ \text{Hours Worked} = 20 \]
Therefore, Mark worked 20 hours.