Write a letter to your friend sharing the experience about 'Dilli darshan trip' from school

'A' writes a letter to his friend 'B' and gives it to his son to post it in a letter box, the reliability of his son being 3/4. The probability that a letter posted will get delivered is 8/9 .At a later date, 'A' hears from 'B' that the letter has not reached him. Find the probability that the son did not post the letter at all .