If you have 20 dice (with 6 faces each), how many different numbers can you create putting the dice in one line and interpreting each face as a digit?
Any of the six digits could appear on the exposed face of a dice, Showing of a face by a dice is arbitrary. So is the appearance of a face in everyone of 20 dices. Hence, the number of 20-digit numbers that could be displayed is 6 X 6 X 6 X .... 20 times. =