`b. 5/7 . 5/11 + 5/7 . 8/11 - 5/7 . 2/11`
`=5/7 .(5/11 + 8/11 - 2/11)`
`=5/7 . 11/11`
`=5/7 . 1=5/7`
`c, 5/13 + (-5)/7 + (-20)/41 + 8/13 - (-21)/41`
`=(5/13 + 8/13)+ ((-20)/41 - (-21)/41) + (-5)/7`
`= 13/13 + (-41)/41 + (-5)/7`
`=1+(-1)+(-5)/7`
`=-5/7`