A straight picket fence is composed of x pickets each of which is 1/2 inch wide. If there are 6 inches of space between each pair of pickets, which of the following represents the length of fence in feet?

A. 13x/2
B. 13x/2 - 6
C. 13x/24
D. (13x+1)/24
E. (13x-12)/24

Equation 
Number of pickeks is x 
Size of pickets = 1/2 
so the length of pickets = 1/2x 
If there are x pickets, it's implied that there are x -1 spaces between the picket 
Length of space = 6 

total number of length = 1/2 x + 6(x-1) in inches 
total lenth in feet =( 1/2 x + 6(x-1))/12 
Simplify (13X-12)/24

OR

VIC plug in
If x = 1, there is only one picket which is 1/2 inch wide. Since 1 foot has 12 inches, 1/2 inch would be 1/24 of a foot.
Look for the choice in which if you put x = 1, you get 1/24.