28.01.2015 Views

A Probability Course for the Actuaries A Preparation for Exam P/1

A Probability Course for the Actuaries A Preparation for Exam P/1

A Probability Course for the Actuaries A Preparation for Exam P/1

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

36 COUNTING AND COMBINATORICS<br />

<strong>Exam</strong>ple 3.5<br />

How many license-plates with 3 letters followed by 3 digits exist if exactly<br />

one of <strong>the</strong> digits is 1<br />

Solution.<br />

In this case, we must pick a place <strong>for</strong> <strong>the</strong> 1 digit, and <strong>the</strong>n <strong>the</strong> remaining<br />

digit places must be populated from <strong>the</strong> digits {0, 2, · · · 9}. A 6-step process:<br />

(1) Choose <strong>the</strong> first letter, (2) choose <strong>the</strong> second letter, (3) choose <strong>the</strong> third<br />

letter, (4) choose which of three positions <strong>the</strong> 1 goes, (5) choose <strong>the</strong> first<br />

of <strong>the</strong> o<strong>the</strong>r digits, and (6) choose <strong>the</strong> second of <strong>the</strong> o<strong>the</strong>r digits. Every<br />

step can be done in a number of ways that does not depend on previous<br />

choices, and each license plate can be specified in this manner. So <strong>the</strong>re are<br />

26 · 26 · 26 · 3 · 9 · 9 = 4, 270, 968 ways

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!