简算：
2又1/17X15-（5又21/23-5又15/17）
=(2+1/17)*15-(5+21/23-5-15/17)
=30+15/17+15/17-21/23
=30+30/17-21/23
=30+(2-4/17)-(1-2/23)
=31+2/23-4/17
=31+(34-92）/(23*17)
=31-58/391
=30又333/391