Question

Adrian's CD player can hold six disks at a time and shuffles all of the albums and their songs. If he has thirteen CD's, how many different combinations of six CD's can he put in the player?

Answer 1

Adrian should get an IPOD.

Answer 2

no time for joking man

Answer 3

how many CDs can you choose from to put in slot 1?

Answer 4

13! / (7! x 6!)

Answer 5

Yeah wolf that's the problem I don't know how to do that I am barely starting the 10th grade tomorrow

Answer 6

If you answer me I help you :)

Answer 7

scot - do you know what a factorial is?

Answer 8

All I see is that it says 6 disks at a time which would mean there are 6 slots so on one slot would be one disk unless I am missing something

Answer 9

nope wolf. This is my first time seeing any factorial.

Answer 10

yes, but my question is, how many CDs do you have to choose from to insert in slot 1?

Answer 11

scot let's make it easy - how many ways can you choose two people from 3 people?

Answer 12

13 Fibona

Answer 13

there are 3 ways to choose 2 people from 3 people (example A B C)
AB
AC and
BC

Answer 14

yup, so now, how many CDs do you have to choose for slot 2 once slot one is filled?

Answer 15

Sorry wolf my test closed I was trying to get it open. You have 12 cd to chose from fibona

Answer 16

and for slot 3? 4? 5? 6?

Answer 17

scot - I wish I knew about the time limit - the answer is 1716

Answer 18

oh wait. sorry, i don't help with tests

Answer 19

well fibona left it was a practice test anyways so yeah

Answer 20

has it timed out? if you still wanna know I'll show you, but not if it's cheating

Answer 21

it's not cheating trust me. It's a VOLUNTARY practice test to get ready for tomorrow

Answer 22

ok

Answer 23

oh and wolf I got it open can you explain how you got that answer?

Answer 24

Here is the formula you would need to get the answer

Answer 25

Yes I have the formula right here but how do you work it out?

Answer 26

Fibonacci - what's the matter?

Answer 27

thinking perms sorry

Answer 28

http://www.mathsisfun.com/combinatorics/combinations-permutations.html
this explains the concept of combinations for you

Answer 29

Okay the n! means ALL the disks availble - in this case 13
and the factorial 13! means 13• 12•11•10•9•8•7•6•5•4•3•2•1

Answer 30

right wolf

Answer 31

the r! (r factorial) means how many elements you are choosing from the 13 - in this case it is 6

Answer 32

I'm following

Answer 33

and (n-r)! means (13-6) ! or 7! (seven factorial)

Answer 34

So we must calculate 13! / (7! • 6!)

Answer 35

13! = 13• 12•11•10•9•8•7•6•5•4•3•2•1
7 ! = 7•6•5•4•3•2•1

Answer 36

Oh I see now. I got the answer you gave me earlier 1716

Answer 37

So 13!/7! =
13• 12•11•10•9•8•7•6•5•4•3•2•1 divided by
7•6•5•4•3•2•1
= 13• 12•11•10•9•8
One more step to go

Answer 38

13• 12•11•10•9•8 / 6!

= 13• 12•11•10•9•8 / 6•5•4•3•2•1

After we do some cancellations we get 1716.
There it is done.

Answer 39

Thanks for staying and explaining.

Answer 40

It's good you also got 1716

Answer 41

it's okay - glad to help out.
By the way here's a combination calculator for you:
http://www.1728.org/combinat.htm

Answer 42

Thanks

Answer 43

okay see you :-)