`P=frac\{1,05-2,1+7/13+7/3}{2,55-5,1+17/13+17/3}`
`=frac\{21/20-21/10+7/13+7/3}{51/20-51/10+17/13+17/3}`
`=frac\{21/20-21/10+21/39+21/9}{51/20-51/10+51/39+51/9}`
`=frac\{21.(1/20-1/20+1/39+1/9)}{51.(1/20-1/10+1/39+1/9)}`
`=21/51`
`=7/17`
`->` Chọn `C`