12 7/23 - (6 18/23-t)=3 21/23+5 19/23
12 7/23 - (6 18/23 - t) = 9 17/23 6 18/23 - t = 12 7/23 - 9 17/23 6 18/23 - t = 2 13/23 t = 6 18/23 - 2 13/23 t = 4 5/23