`b) 1/11 + 2/11 + 3/11 + 4/11 + 5/11 + 6/11 + 7/11 + 8/11 + 9/11 + 10/11`

`= ( 1/11 + 10/11 ) + ( 2/11 + 9/11 ) + ( 3/11 + 8/11 ) + ( 4/11 + 7/11 ) + ( 5/11 + 6/11 )`

`= 1 + 1 + 1 + 1 + 1`

`= 5`

`c) 1/21 + 2/21 + 3/21 + 4/21 + 5/21 + 6/21 + ... + 20/21`

`= ( 1/21 + 20/21 ) + ( 2/21 + 19/21 ) + ( 3/21 + 18/21 ) + ( 4/21 + 17/21 ) + ( 5/21 + 16/21 ) + ( 6/21+ 15/21 ) + ( 7/21 + 14/21 ) + ( 8/21 + 13/21 ) + ( 9/21 + 12/21 ) + ( 10/21 + 11/21 )`

`= 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1+ 1 + 1`

`= 10`

`d) 1/10 + 20/100 + 300/1000 + 4000/10000`

`= 1/10 + 2/10 + 3/10 + 4/10`

`= ( 1 + 2 + 3 + 4 )/10``

`= 10/10`

`= 1`