To find out how many hours the plumber worked, you can divide the total charge by the hourly rate.
The hourly rate is $45, and the total charge is $180.
\[ \text{Hours worked} = \frac{\text{Total Charge}}{\text{Hourly Rate}} = \frac{180}{45} = 4 \]
So, the plumber worked 4 hours.
The correct response is:
4 hours