20－(2/15+1/21)×105
=20-(14+5)
=20-19
=1
17/19×(6/17×2－5/49)
=1/19×(12-17*5/49)
=1/19×(12-85/49)
=(588-85)/17*49
=503/17*49
=503/833
0.125×24×7
=0.125×8×3×7
=1×21
=21