Здравей,

може да разгледаш и този код, мисля че има по-малко проверки и е една идея по-четим ..

boolean validFuel = (fuel.equals("Diesel") || fuel.equals("Gasoline") || fuel.equals("Gas"));
if (!validFuel) {
    System.out.println("Invalid fuel!");
} else {
    if (litres >= 25) {
        System.out.printf("You have enough %s.", fuel.toLowerCase());
    } else {
        System.out.printf("Fill your tank with %s!", fuel.toLowerCase());
        }
    }

Поздрави! :)

import java.util.Scanner;

public class FuelTank {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        // input

        String fueltype = scanner.nextLine();
        int litres = Integer.parseInt(scanner.nextLine());

        if (fueltype.equals("Diesel") || fueltype.equals("Gasoline") || fueltype.equals("Gas")) {
            if (litres >= 25) {
                System.out.println("You have enough " + fueltype.toLowerCase() + ".");
            } else if (litres < 25) {
                System.out.println("Fill your tank with " + fueltype.toLowerCase()+ "!");
            }
        } else System.out.println("Invalid fuel!");
    }
}