There are 12 feet of wrapping paper on each roll. Jill needs 84 feet of wrapping paper. Which equation can Jill use to find the number of rolls, r , she needs? need equation

Let
r----------------- > number of rolls
x-----------------> feet of wrapping paper needs

we know that 
1 r--------------------- > 12 feet of wrapping paper
therefore
r=(1/12)*x------------- > equation required

for x=84 feet of wrapping paper
r=(1/12)*84=7 rolls