If my car's fuel tank fits 18 gallon, how many times do I need to refuel on a 10-hour trip if the average speed is 60mph and the car's fuel efficiency is 30mpg?

Assuming the tank is full when you start, you would have to refuel once. 10 hours at 60mph means the total distance is 600 miles. To drive 600 miles at a fuel efficiency of 30 mpg you would need 20 gallons of fuel. That's more than the 18 gallon capacity of the tank, but less than twice the capacity, so you would need to refuel once.