x+81/19+x+82/18=x+84/16+x+85/15

`(x + 81)/19 +(x + 82)/18 = (x + 84)/16 + (x + 85)/15`

`<=> ((x + 81)/19 + 1) + ((x + 82)/18 + 1) = ((x + 84)/16 + 1) + ((x + 85)/15 + 1)`

`<=> (100 + x)/19 + (100 + x)/18 = (100 + x)/16 + (100 + x)/15`

`<=> (100 + x)/19 + (100 + x)/18 - (100 + x)/16 - (100 + x)/15 = 0`

`<=> (100 + x) . 1/19 + (100 + x) . 1/18 - (100 + x) . 1/16 - (100 + x) . 1/15 = 0`

`<=> (100 + x) . (1/19 + 1/18 - 1/16 - 1/15) = 0`

`<=> (100 + x) . (-287)/13680 = 0`

`<=> 100 + x = 0 : (-287)/13680`

`<=> 100 + x = 0`

`<=> x = 0 - 100`

`<=> x = -100`

`Vậy: x = -100`