Let E1 be the event that letter came from Calcutta and E2 letter came from tatanagar A = event that just two letters TA are visible.
Then P(E1) = 1/2, P(E2) = 1/2
If E1 happens there letter came from calcutta
P(A/E1) = 1/7[TA is assumed as one letter]
If E2 happens then letter came from Tatanagar
P(A/E2) = 2/8
Now using Baye's theorem