A dreidel is a four-sided top that contains a Hebrew letter on each side: nun, gimmel, heh, shin. Here we suppose that a fair dreidel is to be spun ten times. Compute the probability of each of the following events:
a) five gimmels and five hehs.
b) no nuns or shins.
c) two letters are absent and two letters are present.
d) at least two letters are absent.