Question

Mr. A writes a letter to his friend and gives it to his son to post it in a letter box. The reliability of his son is 3/4. The prob that a letter posted will get delivered is 8/9. After few days Mr. A hears from Mr. B that the letter has not reached him. Find the prob. that the son did not post the letter at all.

Answer 1

Probability that letter will reach$=\frac{3}{4}\times \frac{8}{9}=\frac{2}{3}$

Probability that letter will not reach$=\frac{1}{3}$

Probability that son did not post the letter$=\frac{1}{3}-\frac{3}{4}\times \frac{1}{9}=\frac{1}{3}\times \frac{1}{12}=\frac{1}{4}$