10/21 / ( 7/6 - 3/7 )
= ( 42 X 10/21 ) / [ ( 7/6 - 3/7 ) X 42 ]
= ( 2 X 10 ) / ( 7 X 7 - 3 X 6 )
= 20 / ( 49 - 18 )
= 20/31
120 - 3/5 X ( 405 / 27 )
= 120 - 3/5 X 45/3
= 120 - 9
= 111
或者
= 120 - 81 X 3/405 X 405/27
= 120 - 3 X 3
= 120 - 9
= 111120 - 81 X 3/405 X 405/27这个81从哪里来的