After each hour, the amount remaining is 100% - 12% = 0.88 times the amount present at the beginning of the hour. Then after 8 hours, the amount remaining will be the initial amount multiplied by 0.88 eight times, or ...
(120 mg)·(0.88^8) ≈ 43.1561 mg ≈ 43.16 mg
_____
Of course, an exponent is used to signify repeated multiplication. You can actually do the multiplication if you like, but most scientific and graphing calculators handle exponentiation easily.
