`B= 1212/1515 + 1212/3535 + 1212/6363 + 1212/9999`
`B= (12.101)/(15.101) + (12.101)/(35.101) + (12.101)/(63.101) + (12.101)/(99.101)`
`B= 12/15 + 12/35 + 12/63 + 12/99`
`B= 4/5 + 12/35 + 4/21 + 4/33`
`B= 28/35 + 12/35 + 4/21 + 4/33`
`B= 8/7 + 4/21 + 4/33`
`B= 24/21 + 4/21 + 4/33`
`B= 28/21 + 4/33`
`B= 4/3 + 4/33`
`B= 44/33 + 4/33`
`B= 16/11`
Vậy `B = 16/11`