4/5 5/6 6/7 首先通分，找出最大公倍数。即5x6x7=210210/5=42 210/6=35 210/7=30
4/5 5/6 6/7 4/5=4X42/5X42 4/5=168/210 5/6=5X35/6X35 5/6=175/210 6/7=6X30/7X30 6/7=180/210 4/5=168/210 5/6=175/210 6/7=180/210
