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`(x-85)/15 + (x-74)/13 + (x-67)/11 + (x-64)/9 = 10`
`⇒ (x-85)/15 + (x-74)/13 + (x-67)/11 + (x-64)/9 - 10 = 0`
`⇒ ((x-85)/15 - 1) + ((x-74)/13 -2) + ((x-67)/11 - 3) + ((x-64)/9 - 4) = 0`
`⇒ (x-100)/15 + (x-100)/13 + (x-100)/11 + (x-100)/9 = 0`
`⇒ (x-100).(1/15 + 1/13 + 1/11 + 1/9) = 0`
`Vì` `(1/15+1/13 + 1/11 + 1/9) \ne 0`
`⇒ x - 100 = 0`
`⇒ x = 100`
