Answer:
 60 inches
Step-by-step explanation:
Put the given numbers in the formula and solve for the remaining unknown.
 P = 2L +2W
 18 ft = 2L +2×(4 ft)
 10 ft = 2L . . . . . . . . . subtract 8 ft
 5 ft = L . . . . . . . . . . . divide by 2
You want this in inches, so you make use of the fact that each foot is 12 inches.
 5 ft = 5×(12 in) = 60 in
The length of Sarah's fenced yard is 60 inches.
