`a, 3/7 xx 2/5 xx 7/3 xx 20 xx 19/72`
`= ( 3/7 xx 7/3 ) xx ( 2/5 xx 20 xx 19/72 )`
`= 1 xx ( 8 xx 19/72 )`
`= 1 xx 19/9`
`= 19/9`
`b, 6/7 xx 8/13 + 6/13 xx 9/7 - 3/13 xx 6/7`
`= ( 6 xx 8 )/( 7 xx 13 ) + 6/13 xx 9/7 - ( 3 xx 6 )/( 13 xx 7 )`
`= 6/13 xx 8/7 + 6/13 xx 9/7 - 3/7 xx 6/13`
`= 6/13 xx ( 8/7 + 9/7 - 3/7 )`
`= 6/13 xx 2`
`= 12/13`