`A = 1/28 + 1/70 + 1/130 + ... + 1/700`
`A = 1/( 4 . 7 ) + 1/( 7 . 10 ) + ... + 1/( 25 . 28 )`
`A . 3 = 3/( 4 . 7 ) + 3/( 7 . 10 ) + ... + 3/( 25 . 28 )`
`A . 3 = 1/4 - 1/7 + 1/7 - 1/10 + ... + 1/25 - 1/28`
`A . 3 = 1/4 - 1/28`
`A . 3 = 3/14`
`A = 3/14 : 3`
`A = 1/14`