Try this options:
a. total - 6 digits, '6' - 1 digit, then probability of rolling a '6' is 1/6;
b. total - 6 digits, '6' - 1 digit, then probability of rolling 1,2,3,4,5 is 5/6;
c. if probability of rolling a '6' is p and not rolling a '6' is q, then p+q=1;
d. if expected probability of one rolling a '6' is 1/6, then numbers of times of rolling a '6' during 120 times is 120/6=20 times.