`A = 5/( 1 . 2 ) + 5/( 2 . 3 ) + 5/( 3 . 4 ) + ... + 5/( 39 . 40 )`
`A = 5 . ( 1/( 1 . 2 ) + 1/( 2 . 3 ) + 1/( 3 . 4 ) + ... + 1/( 39 . 40 ) )`
`A = 5 . ( 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/39 - 1/40 )`
`A = 5 . ( 1 - 1/40 )`
`A = 5 . 39/40`
`A = 39/8`