Question

Suppose that the probability that your mail is delivered before 2pm is .90. What is the probability that your mail will be delivered before 2pm for two consecutive days? Give your answer as a decimal.

Answer 1

1. The probability that your mail is delivered before 2 pm is 0.9, so the probability that the mail is delivered at 2 pm or after 2 pm (so not before 2 pm) is 1-0.9=0.1.

Remark: 0.9=9/10=90/100=90%, and 0.1 is 10%

2. Probability of the mail being delivered before 2 pm for 2 consecutive days is 
$\frac{90}{100}* \frac{90}{100}= \frac{9}{10}* \frac{9}{10}= \frac{81}{100}=0.81$

3. take a look at the picture attached, the tree diagram is another method we could use.