Ans.
Let E1 denote the event that the letter came from TATA NAGAR and E2 the event that the letter came from CALCUTTA.
Let A denote the event that the two consecutive alphabets visible on the envelope are TA.
Since, the letters have come either from Calcutta or Tata nagar, Therefore,
P (E1) = � and P (E2) = �
If E1 has occurred, then it means that the letter came from TATA NAGAR. In the word TATA NAGAR there are 8 consecutive alphabets. i.e., {TA, AT, TA, AN, NA, AG, GA and AR} and TA occurs two time.
Therefore, P (A|E1) = 2/8=1/4
If E2 has occurred, then it means that the letter came from Calcutta. In the word CALCUTTA there are 7 consecutive alphabets. i.e., {CA, AL, LC, CU, UT, TT and TA} and TA occurs only one time.
Therefore, P (A|E2) = 1/7
P (A|E1) = 2/8, P (A|E2) = 1/7. Therefore, by Bayes theorem we have,
Hence, probability that the letter has come from CALCUTTA is 4/11.
And,
Hence, probability that the letter has come from TATA NAGAR is 7/11.