To find the duration of a plumber's visit that costs $20, we can use the equation given:
\[ c = 20d \]
Here, \( c \) represents the cost, and \( d \) represents the duration in hours. We can substitute \( c = 20 \) into the equation:
\[ 20 = 20d \]
Next, we solve for \( d \) by dividing both sides by 20:
\[ d = \frac{20}{20} \]
\[ d = 1 \]
Therefore, the duration of the plumber's visit that costs $20 is 1 hour.