Answer: Bob used 50% of rope 
 
Explanation:
Let's begin by converting inches to feet 
if 
1foot =12inches 
X foot =18 inches 
Crosss multiplying we have 
X foot =18/12
X=1.5 feet 
18 inches is 1.5feet 
We can also say Bob the builder used 1.5feet of 3 feet of rope 
To get the percentage 
1.5/3*100=
0.5*100=50%