Let x represent the number of ounces of 95% salt she should use.
Then the total amount of salt in the 80 ounces of solution is ...
... 0.95x + 0.70(80-x) = 0.80·80
... 0.25x = 64 -56 . . . . . . . collect terms, subtract 56
... x = 8·4 = 32 . . . . . . . . . . multiply by 4
She should use 32 ounces of 95% salt and (80-32=) 48 ounces of 70% salt,