Math
posted by Kyle .
A contractor requires a cement mix that is 70% sand and 30% cement.
He has 700 kg of a cement mix that is 80% sand and 20% cement.
How much cement should he add so that he gets the required mix?
My set up:
required mix = (S)(.7) + (C)(3)
700kg = (S)(.8) + (C+X)(.2)
Is this even correct?

C / (S+c) = 0.3.
C = 0.8 * 700kg = 560kg.
C / (560+C) = 0.3,
C = 168 + .3C,
C  0.3C = 168,
0.7C = 168,
C = 240kg.