`2021/( 1 . 2 ) + 2021/( 2 . 3 ) + 2021/( 3 . 4 ) + .... + 2021/( 99 . 100 )`
`= 2021 . ( 1/( 1 . 2 ) + 1/( 2 . 3 ) + 1/( 3 . 4 ) + .... + 1/( 99 . 100 ) )`
`= 2021 . ( 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100 )`
`= 2021 . ( 1 - 1/100 )`
`= 2021 . 99/100`
`= 200079/100`