Let F be the 5% borrowing, T=12% borrowing, then 8% borrowing=750000-T-F=5F, because the 8% borrowing was 5 times the 5% borrowing. So 6F+T=$750000 and T=750000-6F.
0.05F+0.08(5F)+0.12T=$68000 interest.
Substitute for T:
0.45F+0.12(75000-6F)=$68000, 0.27F=90000-68000=$22000.
F=22000/0.27 and 8% borrowing=5F=110000/0.27=$407407.41 to the nearest cent.