Hence, the probability that the two coins are of different denomination is:
2/3
Let N denote nickel, D denotes dime and Q denotes Quarter.
Now when two coins are drawn one after the other with replacement then the outcomes is given by:
(N,N) (N,D) (N,Q)
(D,N) (D,D) (D,Q)
(Q,N) (Q,D) (Q,Q)
This means that there are a total of 9 outcomes.
The outcomes such that both the denominations are different i.e. the number of favorable outcomes are: 6
{ (N,D) (N,Q) (D,N) (D,Q) (Q,N) (Q,D) }
The probability that the two coins are of different denomination is:
6/9=2/3