Answer:
11/18
Explanation:
The desired probability is the sum of ...
... (probability of choosing a coin) × (p(heads) on that coin)
Since the coins are chosen at random, we assume the probability of choosing a given coin is 1/3. Then ...
... p(heads) = (1/3)·(1/2) + (1/3)·1 + (1/3)·(1/3) = 1/6 + 1/3 + 1/9 = (3 +6 + 2)/18
... p(heads) = 11/18