Let x represent the amount invested at the higher rate. Then 70,000-x is the amount invested at the lower rate. The interest earned is ...
... 0.13x + 0.05(70000-x) = 6000
... 0.08x + 3500 = 6000 . . . . . . . . . simplify
... .08x = 2500 . . . . . . . . . . . . . . . . . subtract 3500
... x = 2500/0.08 = 31250 . . . . . . . .divide by .08
Betsy should invest $31,250 at 13% and $38,750 at 5%.