Unformatted text preview: alphabet? Out of 26 letters, four are to be chosen at a time. Remember that the order is very important, since each order stands for a diﬀerent word. This is a clear case for using permutation. Hence the number of words that can be formed in this case is P (26 , 4). This is equal to 26! / 22!, which is 23 · 24 · 25 · 26. So the number of words is a phenomenal 358,800. 3. (7 pts) An urn contains 15 diﬀerent balls. In how many ways can you select four balls without replacement? Four balls can be selected out of ﬁfteen balls in C (15 , 4) ways. Applying the formula for combinations, 15! ÷ (4! · 11!) = 12 · 13 · 14 · 15 ÷ (2 · 3 · 4)=1365 ways....
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 Spring '09
 Lewis
 Math, Timothy Lewis TA

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