SOLUTION: How many liters of a 7% solution of salt should be added to a 27% solution in order to obtain 780 liters of a 13% solution?

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Question 750796: How many liters of a 7% solution of salt should be added to a 27% solution in order to obtain 780 liters of a 13% solution?
Answer by josgarithmetic(39617) About Me  (Show Source):
You can put this solution on YOUR website!
(amount of acid)/(amount of mixture)=fraction as the acid or the concentration

Some parts are variable.

Let u = liters volume to add of the 7% acid.
Let v = liters of 27% acid.

Amount of acid = u*7%+v*27%
%287u%2B27v%29%2F780=13 and u%2Bv=780
Although one of these is rational equation, they both are linear equations in two variables.
Just solve the system for u and v.