39/80乘以45/51加上31/80乘以6/51
=39/80*45/51+31/80*6/51
=（8/80+31/80）*45/51+31/80*6/51
=8/80*45/51+31/80*45/51+31/80*6/51
=8/80*45/51+31/80*（45/51+6/51）
=1/10*45/51+31/80
=3/34+31/80
=647/1360