# 1)In each of the following scenarios, tell whether permutations (ordered) or combinations (unordered) are being described. Explain your answer. (3 pts.) a. A president, vice-president, and...

1)In each of the following scenarios, tell whether permutations (ordered) or combinations (unordered) are being described. Explain your answer. (3 pts.)

a. A president, vice-president, and secretary are chosen from a 25-member garden club.

b. A cook chooses 5 potatoes from a bag of 12 potatoes to make a potato salad.

c. A teacher makes a seating chart for 22 students in a classroom of 30 desks.

2) There are 680 three-digit numbers that are available for use as area codes in North America. As of April 2010, 301 of them were actually being used.

a. How many additional three-digit codes are available for use?

b. Within a given area code, how many unique telephone numbers are theoretically possible?

c. As of April 2010, how many total phone numbers are possible in North America?

3)How many 9-letter “words” can be formed from the letters of the word “logarithm”?

4)Professor Dupré Calc gives her class 20 study questions, from which she will select 8 to be on the final exam. How many ways can she select the questions?

5)Find the coefficient of the x^11y^3 term in the binomial expansion of (x+y)^14.

6)Use the Binomial Theorem to expand the expression (2x-3y)^5

7)Sal opens a box of a dozen chocolate cremes and generously offers two of them to his friend Val. Val likes vanilla cremes the best, but all of the chocolates look alike on the outside. If four of the cremes are vanilla, what is the probability that both of Val’s picks turn out to be vanilla?

8)Michael makes 90% of his free throws in basketball. If he shoots 20 free throws, and if his chance of making each one is independent of the other shots, what is the probability that he make?

a.all 20?

b.exactly 18?

c.at least 18?

9)In the game Yahtzee, on the first roll five dice are tossed simultaneously. What is the probability of rolling five of a kind (also known as “Yahtzee”) on the first roll?

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### 1 Answer

**1)**

**a.** Permutations

Let's say we are first choosing a president, then vice-president and then secretary. We can choose president in 25 ways (1 out of 25 members). Now we have only 24 members to choose from so we can choose a vice-president in 24 ways and analogously we can choose secretary in 23 ways. Hence 25*24*23=13800. If we were choosing 3 members for the same function then we would have combinations because there is no way to differentiate them.

**b. **Combinations

Because potatoes are all the same (mathematically speaking) so their order is not important. `((12),(5))=792`

**c. **Permutations

First student can be seated at any of 30 desks, second student can be seated at any of 29 remaining desks etc. Order does matter because it is not the same weather John or Jane sits at the first desk. ` prod_(i=9)^30 i=9*10*11*cdots*29*30=6578691959627754430464000000`

**2)**

**a.** 680-301=379

**b. **If a phone number has 7 digits (without area code) then there are `10^7=10000000` possible phone numbers in a given area. Each of the 7 digits can be 0,1,...,8,9.

**c. `10^7` **per area code times 301 area codes equals 3010000000.

**3)**

This is the same as asking how many permutations of those 9 letters are there. Obviously order of letters in a word matters. `9! =362880`

**4) **Order of questions doesn't matter only if a question is chosen or not. `((20),(8))=125970`

**5) `((14),(3))=364` **Lower number is exponent of y.

**6)** `(2x-3y)^5=((5),(0))(2x)^5-((5),(1))(2x)^4(3y)^1+((5),(2))(2x)^3(3y)^2-((5),(3))(2x)^2(3y)^3+((5),(4))(2x)^1(3y)^4-((5),(5))(3y)^5=`

`=32x^5-240x^4y+720x^3y^2-1080x^2y^3+810xy^4-243y^5`

**7)**

She can choose 2 vanilla chocolate in `((4),(2))=6` ways and she can choose 2 chocolate from 12 in `((12),(2))=66` ways. Hence probability is `6/66=1/11.` You could use permutations here as well but you would have to calculate with bigger numbers, of course result would be the same.

**8) **

**a.** 90%=0.9

`0.9*0.9*cdots*0.9=0.9^20approx0.12157665`

**b.** `0.9^18*0.1^2approx0.0015`

**c. **`0.9^18*0.1^2+0.9^19*0.1+0.9^20approx0.136586`

**9)** To simplify the task let's say the dices are rolled one by one (this actually doesn't matter but it's useful for better understanding of the problem). First dice is rolled and we get some number so we must get the same number an all four of the remaining dices and probability of getting a particular number when rolling a dice is `1/6` . Hence we have

`1/6*1/6*1/6*1/6=1/1296`

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