`(3x)/(2.5) + (3x)/(5.8) +(3x)/(8.11) + (3x)/(11.14) = 1/21`
`=> x . (3/(2.5) + 3/(5.8) + 3/(8.11) + 3/(11.14)) = 1/21`
`=> x . (1/2 - 1/5 + 1/5 - 1/8 + 1/8 - 1/11 + 1/11 - 1/14) = 1/21`
`=> x . (1/2 - 1/14) = 1/21`
`=> x . 3/7 = 1/21`
`=> x = 1/21 : 3/7`
`=> x = 1/9`
Vậy `x = 1/9`