change all 3 to improper fractions
so 5 1/27 = 5(27)+1 all over 27 = 136/27
3 7/8 = 3(8) + 7 all over 8 = 31/8
12 7/9 = 12(9) + 7 all over 9 = 115/9
now need a common denominator, which will be 27(8) = 216
so, 136/27 needs to be multiplied by (8/8) = 1088/216
31/8 needs to be multiplied by (27/27) = 537/216
and 115/9 gets multiplied by (24/24) = 2760/216
Add all numerators (1088 + 537 + 2760) = 4385
last divide by 216 to get mixed number = 20 65/216