解：设$20 \mathrm {mL}$样品果汁中的维生素$\mathrm {C}$的质量为$x。$
$\mathrm {C}_6\mathrm {H}_8\mathrm {O}_6+\mathrm {I}_2\xlongequal[ ]{ }\mathrm {C}_6\mathrm {H}_6\mathrm {O}_6+2\mathrm {HI} $
176 254
$x$ 12.7 mg
$\frac {176}{254}=\frac {x}{12.7 \mathrm {mg}}，$解得$x=8.8 \mathrm {mg}。$
故$100 \mathrm {mL}$该果汁中维生素$\mathrm {C}$的含量：$8.8 \mathrm {mg}×100 \mathrm {mL}/20 \mathrm {mL}=44 {\mathrm {mg}}＜50 \mathrm {mg}$
答：该果汁中维生素$\mathrm {C}$的含量与包装盒上标明的维生素$\mathrm {C}$的含量不相符。
