$\dfrac{x-85}{15}+\dfrac{x-74}{13}+\dfrac{x-67}{11}+\dfrac{x-64}{9}=10$
`<=>` $\left({\dfrac{x-85}{15}-1}\right)+\left({\dfrac{x-74}{13}-2}\right)+\left({\dfrac{x-67}{11}-3}\right)+\left({\dfrac{x-64}{9}-4}\right)=0$
`<=>` $\dfrac{x-100}{15}+\dfrac{x-100}{13}+\dfrac{x-100}{11}+\dfrac{x-100}{9}=0$
`<=>` $(x-100).\left({\dfrac{1}{15}+\dfrac{1}{13}+\dfrac{1}{11}+\dfrac{1}{9}}\right)=0$
`<=>` $x-100=0.$ (Vì $\dfrac{1}{13}+\dfrac{1}{11}+\dfrac{1}{9}\neq0$)
`<=>`$x=100$
