Posted by **Kyle** on Thursday, August 25, 2011 at 3:10pm.

**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?

- Math -
**Henry**, Thursday, August 25, 2011 at 4:27pm
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.

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