confusing math question help please!

Question

how many liters of an 80% acid solution must be mixed with a 15% acid solution to get 390L of a 70% acid solution?

Answer 1

Let E=number of liters of 80% acid solution.

Let F=number of liters of 15% acid solution.

 

   E+    F=390                 total amount of solution

.8E+.15F=.7×390=273   total amount of acid

 

multiply first equation by .8    .8E+.8F  =312

                                            .8E+.15F=273

subtract                                        .65F=39  gives F=60

and the first equation then gives      E=330