Let Xiaohong do the X problem, then Xiaoming does the X+8 problem and Xiao Fang does the 72-X-(X+8) = 64-2x problem.

X+(X+8)-(64-2X)= 12

4X=68

X= 17

X+8= 17+8=25

64-2X=64- 17*2=30

Xiaohong did 17, Xiaoming did 25 and Xiao Fang did 30.