Let us calculate the area of the walk.
Since the width of the walk is 3 feet, and the walk will be placed all the 4 sides,
Length of the plot including walk = 20 + (2 × 3) = 20 + 6 = 26 feet.
Breadth of the plot including walk = 30 + (2 × 3) = 36 feet.
Area of the plot including walk = 26 × 36 square feet.
Area of the plot = 20 × 30 square feet.
Area of the walk = (26 × 36) - (20 × 30)
= 936 - 600
= 336 square feet.
= square yards
= 37.3 square yards
Cost of the walk per square yard = $12.25.
Therefore, total cost of the walk = 12.25 × 37.3 = $456.92.