It is given that the trip is 420 miles and the car averages 20 miles per galon of gas.
The cost of gasoline is $2.95 per gallon.
First calculate the number of gallons of gas required for the trip, let it be g, so it follows:

Hence 21 gallons of gas is required for the trip.
Let the cost of gasoline be c so it follows:

Hence $61.95 will be the gasoline cost for the trip.