Urns I, II and III contain three pennies and four dimes, two pennies and five dimes and three pennies and one dime, respectively. One coin is selected at random from each urn.
(a) What is the probability that all three selected coins have the same denomination?
P(a:same denomination)=(3/7)(2/7)(3/7)+(4/7)(5/7)(1/4)= 19/98
(b) If two of the three coins are dimes, what is the probability that the coin selected from urn I was a dime?
My concern: I have no clue where I am suppose to start to solve this exercise, any help will be apprecited. Thanks!