Any of 6 given digits in the left-most position, giving 6 options;


any of remaining 5 digits in the next position, giving 5 options;


any of remaining 4 digits in the next position, giving 4 options;


any of remaining 3 digits in the last position, giving 3 options.


The ANSWER is the product of the number of options in each position


    6*5*4*3 = 360 different numbers.