Answer: The family budget to make the one-way trip should be $70.
Step-by-step explanation:
Given: The cost of three gallons of gasoline=$10.50
Therefore, the cost of one gallon of gasoline=
Since, they need 20 gallons for driving out of state to her grandmother’s house.
The cost of 20 gallons of gasoline = 
Therefore, The family budget to make the one-way trip should be $70.