Question 613620
following the tossing instructions, there are six possible outcomes
___ hHH, hHT, hTH, hTT, tH, tT 


there are two ways to get 2 tails ___ hTT or tT


like the basketball problem, the probability of any string is the product of the individual probabilities


hTT __ 3/5 * 1/3 * 1/3 = 1/15


tT ___ 2/5 * 1/3 = 2/15


these are two of the possible strings, so their probabilities add to give the probability for the 2 tail combination


probability of two tails ___ 1/15 + 2/15 = 3/15 = 1/5 = .2