Please check the wording of the problem.
As written, the problem is truly silly . . .
One ounce of the mixture containing 6% salt
is to be mixed with two ounces of a mixture which is 15% salt
in order to obtain a solution which is 12% salt. . This is true.
What is the percentage of salt in the resulting solution?
I'll take a wild guess . . . um . . . twelve percent?