The formual for total cost of driving cars rented from companies A,B,and C must be determined.

So let,

x - be the number of miles the car travels.

y - the number of days the caris rented and

Z - the total cost of driving rented cars from each company.

Since the total cost depends on the miles travelled by the car and the days rented, the genral formula is:

Total Cost=(Cost per mile)(total miles travelled) + (Cost per day)(# of days rented)

For Company A, the given cost per mile is $0.20 and the cost per day is $20. Substituting this to the formula of total cost yields:

`Z_A=0.20x + 20y`

For company B, the cost per mile is $0.10 and the rent per day is $35. So the total cost of cars rented from B is:

`Z_B= 0.10x + 35y`

Note that company C has no mileage charge. So its cost per mile is $0. And this indicates that the total cost of renting a car in company C depends only in the cost per day which is $70.

`Z_C= 0x + 70y`

`Z_C=70y`

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**Hence the three formulas for the total cost of driving rented cars from A,B and C are:**

**`Z_A=0.20x+20y` , `Z_B=0.10x+35y` , `Z_C=70y` **