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**I-Think** Are you sure? Talked to a friend and he said the answer was 100. He didn't give reasons, just hints.

Checking at $\displaystyle v=10$, we have $\displaystyle f(v)=9900$, so that is for every $\displaystyle 10km$ traveled in an hour, $\displaystyle 9900ml$ of fuel is consumed, giving us a consumption rate of $\displaystyle 990mlkm^{-1}$

But at $\displaystyle v=100$, we have $\displaystyle f(v)=8100+9900=18000$, that is for every $\displaystyle 100km$ traveled in an hour, $\displaystyle 18000ml$ of fuel is consumed, giving us a consumption rate of $\displaystyle 180mlkm^{-1}$

Is there something wrong with this reasoning?