SOLUTION: A certain concrete mixture contains 4.00% cement and 5.00% sand. How many kilograms of this mixture and how many kilograms of sand should be combined with 225 kg of cement to

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Question 1069196: A certain concrete mixture contains 4.00% cement and 5.00% sand.

How many kilograms of this mixture and how many kilograms of sand should be combined with 225 kg of cement to make a batch that is 13.0% cement and 14.0% sand? Carry out all calculations exactly and round to three significant digits final answers only.
Answer by josgarithmetic(39617) About Me  (Show Source):
You can put this solution on YOUR website!
The concentrations here are shown as decimal fractions.
x, amount of concrete mixture to add
y, amount of sand to add
    CONCRETE MIXTURE         CEMENT            SAND          TOTAL MASS

           x                 225               y             x+225+y

    ----------------------------------------------------------------------------

                          0.04x+225           0.05x+y

CONC.s                     0.13               0.14
WANTED

Setup the equations for the desired concentration of each material.

Solve for x and y.