Question 1069196
The concentrations here are shown as decimal fractions.
x, amount of concrete mixture to add
y, amount of sand to add
<pre>
    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
</pre>

Setup the equations for the desired concentration of each material.

{{{system((0.04x+225)/(x+y+225)=0.13,and,(0.05x+y)/(x+y+225)=0.14)}}}

Solve for x and y.