For this case, the first thing we must do is define variables:
 x: rental charge per day
 y: rental charge per mile
 We write the system of equations:
 3x + 300y = 81
 5x + 600y = 145
 Solving the system we have:
 x = 17
 y = 1/10
 Answer:
 rental charge per day = 17 $
 rental charge per mile = 1/10 $