A rent-a-car charged a fixed daily rate plus a fixed amount per kilometer. A total charge for the first week was $830 with 1250 kilometer on the clock. The second week bill was $710 with 850 kilometer on the clock.Determine the rental charge and the cost per kilometer?

Answer 1

1250x + f = 830
850x + f = 710

or, note that it cost an extra

$120 for 400 km.
That means the mileage rate is $0.30/km

use that to get the fixed rate.

Answer 2

1250x + f = 830
850x + f = 710