An unbiased coin is tossed 3 times, if the third toss gets head what is the probability of getting at least one more head?

Concept: 

• The number of ways for selecting r from a group of n (n > r) = nCr 
• The probability of particular case = \(\rm \text{Number of ways for the case can be executed}\over{\text{Total number of ways for selection}}\)

Calculation

If it is known that third toss gets head, the possible cases:

(H, H, H), (H, T, H), (T, H, H), (T, T, H)

∴ Total cases possible = 4

Total favourable cases = 3 [(H, H, H), (H, T, H), (T, H, H)]

So, required probability P = \(\rm \text{Total favorable cases}\over\text{Total possible cases}\)

P = \(3\over4\)