elimination and substitution word problem

Hi guys , i am in grade 10 gifted math and am doing a series of word problems that involve elimination and substitution. I am good at math , but am still a bit ESL , so am having trouble decoding the first 2 equations that i am supposed to have.

The question is : A car averages 8.5L/100km in city driving and 6.3L/100km on the highway. In 720km of driving , it used 55.7L of fuel.

a)How far was the car driven in the city?

b)How much fuel did it use in city driving?

From what i read i figured that i would let the distance driven in the city be c km , and the distance driven in the highway be h km

The first equation in my opinion should be

c+h=720

I am unsure however on what the second equation should be.

I tried 8.5c+6.3H=720(55.7)

and my end-result of c , was over 1900. Obviously i made a mistake there x(.

i was thinking of doing it as 8.5c/100+ 6.3h/100= 720(55.7)/100 but this would simply give me the same wrong answer.

Any help would be much appreciated