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 Post subject: Veritas Prep Combinatorics Guide Page 15Posted: Fri Jul 15, 2011 8:43 am

Joined: Fri Jul 15, 2011 8:40 am
Posts: 1
#1: Bill is renting DVDs from the video store. He must choose 3 DVDs from a list of 8 new releases and then decide in what order to watch them. how many different schedules of videos can he create, if he cannot watch the same movie more than once?

a) 24
b) 56
c) 72
d) 336
e) 512

I don't get why the answer is 336. I understand that there are 8 dvds to choose from for the first of three dvd then 7 dvds to choose from then 6 dvds to choose from resulting in 8*7*6. What confused me is "and then decide in what order to watch them." part.

The 3 dvds can be ordered in 3*2*1 = 6

So shouldn't the overall possibilities be 336 * 6?

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 Post subject: Re: Veritas Prep Combinatorics Guide Page 15Posted: Sat Jul 16, 2011 11:14 am

Joined: Thu Feb 12, 2009 6:32 pm
Posts: 497
By following the method "8 choices for the first", "7 choices for the second", "6 choices for the third" we are accounting for the order in which the DVD's will be watched.

-- Veritas Help

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 Post subject: Re: Veritas Prep Combinatorics Guide Page 15Posted: Sun Jul 17, 2011 7:27 am

Joined: Thu Feb 12, 2009 6:32 pm
Posts: 497
If we were simply choosing 3 dvds from 8 and the order did not matter we would use the combinations formula.

8!/(3!(8-3)!)

= 8*7*6/(3*2*1) = 56 ways to choose 3 from 8

Once we have our 3 selected, we have 3*2*1 = 6 ways to sequence how we watch them.

- Veritas Help

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