Circumference of the patio is πd, where d is the diameter, so the circumference measures 18π feet=216π inches (about 56.55 feet). Each light strand measures 54 inches, so he would need 216π/54=4π=12.57. 12 strands would cover 54 feet, so there would be a gap of 56.55-54=2.55 feet. 13 strands would be too much. The best solution is to spread the gap equally between the strands. So divide 216π by 12=18π=56.55 inches. So the gap between each strand would need to be 2.55 inches. 12×2.55in=2.55ft, which is the total size of the gap.