Answer:
c=3t is the equation
18 cubic yards of concrete is what we need for a 6 inch thick bridge.
Step-by-step explanation:
If something directly varies, that means there is a constant, k, such that when you multiply it to one variable you always get the other variable.
That is for this particular problem we have c=kt where c is cubic yards and t is thickness in inches.
So we are given from the first sentence: 12 cubic yards and 4 inches thick which means we have:
12=k(4)
Divide both sides by 4:
12/4=k
Simplify:
3=k
So the equation is c=3t for any point (c,t) in this relation.
So we want to know many cubic yards of concrete (we want to know (c) so that Dru can build a 6 inch (t) thick bridge.
c=3(6)
c=18
18 cubic yards