`20/(5xx7) + 20/(7xx9) + 20/(9xx11) +...+ 20/(33xx35)`
`= 20 xx ( 1/(5xx7) + 1/(7xx9) + 1/(9xx11)+...+ 1/(33xx35))`
`= 10 xx 2( 1/(5xx7) + 1/(7xx9) + 1/(9xx11)+...+ 1/(33xx35))`
`= 10 xx ( 2/(5xx7) + 2/(7xx9) + 2/(9xx11) +...+ 2/(33xx35))`
`= 10 xx ( 1/5- 1/7 + 1/7 - 1/9+...+ 1/33- 1/35)`
`= 10 xx (1/5- 1/35)`
`= 10 xx 6/35`
`= 12/7`