\(\$3.20\)
Explanation
The perimeter of the garden is \(20 \times 4=80\) feet, so you would need \(5 \times 80=400\) feet to go around it five times. Now, divide 400 feet by 50 feet and you get 8, which means that you need to buy 8 spools. Finally, 8 spools at 40 cents a spool would cost \(\$3.20\).