Argument
The only number he can't roll is the number one. There are 5 numbers that he can roll.
The probability of success therefore is 5/6.
Formula
Number of successful rolls = probability of success * total number of rolls.
Total number of rolls = 78
Probability = 5/6
Number of successful Rolls = 5/6 * 78
Number of successful Rolls = 390/6
Number of successful Rolls = 65
Answer
He should roll a number greater than one 65 times <<<<< answer