From an 8%, a 12% and a 20% you could make up:
8 + 12 = 20 .... 20 / 2 = 10%
12 + 20 = 32 ... 32 / 2 = 16%
8 + 20 = 28 .... 28 / 2 = 14%
then you could mix any of those percents you have made with any other percent to make more options:
8 + 16 = 24 ... 24 / 2 = 12%
16 + 20 =36 ... 36 / 2 = 18%