For this case we have that the cost for each gallon of gasoline is given by:
[tex]\frac {10.5} {3} = 3.5[/tex]
We have that the total number of gallons is equal to 20.
As the family is going to make only one trip then, the total cost is given by:
[tex]20 * 3.5 = 70[/tex]
Answer:
the family should budget 70 $ to make the one-way trip