Answer:
10 gallons of gas
Explanation:
If John's car can travel 40 miles on 4 gallons of gas, then it can travel 10 miles on 1 gallon of gas.
To travel 100 miles, the car would need 100/10 = 10 gallons of gas.
Since it cost $30 for 4 gallons of gas, 1 gallon of gas would cost $30/4 = $7.50.
Therefore, it would cost 10 x $7.50 = $75 for the car to travel 100 miles.