Answer the problem came with—cogent reason:
Flip the coin twice.
HH repeat two flips
HT grade is A
TH grade is B
TT repeat two flips
Comment by a friend to whom I gave P. I—insufficient reason:
This solution is unnecessary. On Day One, the students had no reason to expect that the coin—if unfair—would have either particular bias. Independently, they had equally no reason to expect either particular assignment of heads and tails. Therefore, on the day the students were required to decide whether or not to take the course for credit, the prospect of a simple single flip sufficed to guarantee the promised outcome.
(“Cogent reason” and “insufficient reason” refer to the two extreme philosophies for the assignment of probability. (But you knew that?))