`99/( 1 . 5 ) + 99/( 5 . 9 ) + 99/( 9 . 13 ) + ... + 99/( 95 . 99 )`
`= 99/4 . ( 4/(1 . 5 ) + 4/( 5 . 9 ) + 4/( 9 . 13 ) + ... + 4/( 95 . 99 ) ) `
`= 99/4 . ( 1/1 - 1/5 + 1/5 - 1/9 + 1/9 - 1/11 + ... + 1/95 - 1/99 )`
`= 99/4 . ( 1 - 1/99 )`
`= 99/4 . 98/99`
`= 98/4 = 24,5` ( Điều phải chứng minh )