` (1/ 15 + 1/35 + 1/63+ 1/99 + 1/143) . x = (20/19 - 20/29 + 20/59)/(39/19 - 39/29 +39/59)`
`=> (1/ (3.5) + 1/(5.7) + 1/(7.9) + 1/(9.11) + 1/(11.13)) . x = (20 .(1/19 - 1/29 + 1/59))/(39. (1/19 -1/29 + 1/59))`
`=> [1/2 .(2/ (3.5) + 2/(5.7) + 2/(7.9) + 2/(9.11) + 2/(11.13))] . x = 20/39`
`=> [ 1/2 . (1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + 1/9 - 1/11 + 1/11 - 1/13)] . x = 20/39`
`=> [1/2 . (1/3 - 1/13)] . x = 20/39`
`=> (1/2 . 10/39) . x = 20/39`
`=> 5/39 . x = 20/39`
`=> x = 20/39 : 5/39`
`=> x = 4`
Vậy `x=4`
