Question

<b>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?</b>

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.


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