A Letter Problem

Probability Level 3

A writes a letter to his friend B and gives it to his son to post it in a letterbox , the reliability of the son being 34 \frac{3}{4} . The probability that a letter posted will get delivered at all is 89 \frac{8}{9} . At a later date A hears from B that the letter had not reached him.

Find the probability that the son did not post the letter at all.

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