8/21*7/8-7/8*5/21
=7/8*(8/21-5/21)
=7/8*3/21
=7/8*1/7
=1/8
11/23*24
=11/23*(23+1)
=11/23*23+11/23*1
=11+11/23
=11又11/23
100*5/98
=(98+2)*5/98
=98*5/98+2*5/98
=5+10/98
=5又5/49
8/21*7/8-7/8*5/21
=7/8*(8/21-5/21)
=7/8*3/21
=7/8*1/7
=1/8
11/23*24
=11/23*(23+1)
=11/23*23+11/23*1
=11+11/23
=11又11/23
100*5/98
=(98+2)*5/98
=98*5/98+2*5/98
=5+10/98
=5又5/49