Precalculus (2018 edition)
Thinking about how many ways you can pick four colors from a group of 6. Created by Sal Khan and Monterey Institute for Technology and Education.
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- What if the colours CAN repeat? How would the process/equation change?(25 votes)
- 6 x 6 x 6 x 6
Think about it like this: Since the colours can repeat and there are 6 alternatives, there will be 6 possible colours for every slot.(63 votes)
- Am not much into binary numbers and such but am wondering... Why can we store 256 different values into 8 bits and why does 8 bit translate into a decimal value of 4 decimal spaces? I know that an 8 bit color has 256 levels of black and white. And I can get any color when I mix 3 basic colors, Red, Green and Blue and using combinatronics I get 256^3 since these 3 colors can have same value or same level of shade between black and white. But why does 8 bit translate to 256 shades?(9 votes)
- Lets suppose you have just 2 bits. How many numbers can you represent?
0 0 = 0
0 1 = 1
1 0 = 2
1 1 = 3
So with just 2 bits we can represent 4 numbers. Notice that 2^2 = 4
Let's add another bit, now how many numbers can we represent?
0 0 0 = 0
0 0 1 = 1
0 1 0 = 2
0 1 1 = 3
1 0 0 = 4
1 0 1 = 5
1 1 0 = 6
1 1 1 = 7
So with three bits we can represent 8 numbers, 0 to 7. Notice that 2^3 = 8.
That is the pattern. So, with 8 bits we can represent 2^8=256 different numbers or levels.(26 votes)
- in how many ways can the first, second, and third placers be chosen from a group of 8 contestants?(5 votes)
- Sorry i'm replying to you 8 years later, but it's better than nothing.
If you watch the Permutation formula video, you see that if you don't have enough spots for every position, you take the places, which there are 3 in this case, and then start from 8 and count down 3, if it was 4 you would count down 4, etc. So 8,7, and 6. So you multiply 8 7 and 6. And you get 336 ways that first second and third placers be chosen from 8 people.(4 votes)
- can anyone tell me why the factorial of 0 is 1?(5 votes)
- There's only one way to do nothing hence 0! =1, like the number of ways to arrange 4 objects is 4! the number of ways to arrange 0 objects is 1, by doing nothing.(2 votes)
- And what if BRYG is considered the same as GRYB? What numbers are input into the spaces then, for calculating the answer?(4 votes)
- Good question! He mentions that because he considers them different, he is finding permutations (P). When you consider BRYG and GRYB the same, that is called finding the number of combinations (C). You can see that there will (almost) always be fewer combinations than permutations, since lots of permutations will only count as a single combination. If you are choosing r things out a collection of n things (in this case we are choosing 4 colors out of a total of 6 colors, so r = 4 and n = 6), then C(n, r) = P(n, r) / r!, where the "!" means "factorial" (3! = 3*2*1 = 6, 5! = 5*4*3*2*1 = 120, etc.). So in this problem, the number of combinations would equal the number of permutations divided by 4! = 4*3*2*1 = 24. So if BRYG, GRYB, RGBY, etc., were considered the same, then instead of 360, the number of possibilities would be 360/24 = 15. Not very many!(6 votes)
- this is a question about homework but this is not the real question: If the numbers 1-8 are used to make 3-digit numbers and they do not reapt , how many 3-digit numbers can be made? What is the formula? I don't get this. :((4 votes)
- Try the multiplication principle. You have 8 choices for the first digit, then (because there are no repeats) 7 choices for the second digit, and so on. Multiply 8*7*6 to get your answer. This is also the "permutation" formula, however: 8 P 3 = 8!/(8-3)!.(5 votes)
- in how many ways can 30 people be divided into 15 couples(4 votes)
- If you have 2 people and 1 couple, then there is only 1 option.
If you have 4 people and 2 couples, then the first person will select from 3 people. 2 people are left after they split off, but there is only 1 way to combine 2 people so we get 3x1.
If you have 6 people and 3 couples, then the first person chooses from 5 people and that leaves 4 people to make 2 couples. From above this is 3x1. So all together this makes 5x3x1
You can keep going like this and you'll see that you end up with a factorial of odd numbers up through 29. To get this, you can divide 30! by the factorial of even numbers through 30. This looks like (2^15)x15! because (2x2x2...2)(1x2x3....15) = (2x4x6...30). So all together it would look like
30! / [(2^15) x 15!] = 6.19 x 10^15(3 votes)
- I have an interesting problem on combinatorics for football (soccer) lovers...
Try to work out how many games are played in one season of the Barclays Premier League... (in England)
[For those of you who aren't football lovers, here are some clues:
1.There are 20 teams in the Premier League.
2.Each team plays every other team twice, once at home and once away.]
P.S. I was watching a match and suddenly wondered how many matches are played in total - that's one of the reasons why I wanted to learn about this topic....(3 votes)
- How can I defferentiate a permutation problem from a combination problem? How can I know how formula I should use?(2 votes)
- Permutations and combinations are similar. When order matters, it's a permutation. When order doesn't matter, it's a combination.(3 votes)
- What if we neglected order, how many combinations would there be and what bis the process to figure it out?(2 votes)
- If we neglected order, then we would be using combination. The combinations would be depending on the limit of things you can choose, and how many things you have to choose from. In the video above, the limit would be 4 and the amount of things to choose from is 6. The process to figure out the number of choices is t!/(t-l)!, where 't' is the number of things you can choose from and 'l' is the limit of things you can choose. Using the numbers from the video, it would be 6!/(6-4)!, which would be 6!/2!, which would be 6*5*4*3*2*1/2*1. The 2*1 would cancel, so you are left with 6*5*4*3. This would be 360 combinations that you can make using 4 items from 6 items. In your scientific calculator, there is a button that marks nCr, and nPr, each referring to combination and permutation respectively. To figure out the answer above, all you have to do is press 6, then nCr, then 4, and enter, and you should end up with 360. Same thing for permutations. Remember that the first number is the items you can choose from, and the second is the limit that you were given.(3 votes)
In one game, a code made using different colors is created by one player, the codemaker, and the other player, the codebreaker, tries to guess the code. The codemaker gives hints about whether the colors are correct and in the right position. All right. The possible colors are blue-- let me underline these in the actual colors-- blue, yellow, white, red, orange and green. Green is already written in green, but I'll underline it in green again. And green. How many 4-color codes can be made if the colors cannot be repeated? To some degree, this whole paragraph in the beginning doesn't even matter. If we're just choosing from-- let's see, we're choosing from-- how many colors are there? There's 1, 2, 3, 4, 5, 6 colors, and we're going to pick 4 of them. How many 4-color codes can be made if the colors cannot be repeated? And since these are codes, we're going to assume that blue, red, yellow and green, that this-- that that is different than green, red, yellow and blue. We're going to assume that these are not the same code. Even though we've picked the same 4 colors, we're going to assume that these are 2 different codes, and that makes sense because we're dealing with codes. So these are different codes. So this would count as 2 different codes right here, even though we've picked the same actual colors. The same 4 colors, we've picked them in different orders. Now, with that out of the way, let's think about how many different ways we can pick 4 colors. So let's say we have 4 slots here. 1 slot, 2 slot, 3 slot and 4 slots. And at first, we care only about, how many ways can we pick a color for that slot right there, that first slot? We haven't picked any colors yet. Well, we have 6 possible colors, 1, 2, 3, 4, 5, 6. So there's going to be 6 different possibilities for this slot right there. So let's put a 6 right there. Now, they told us that the colors cannot be repeated, so whatever color is in this slot, we're going to take it out of the possible colors. So now that we've taken that color out, how many possibilities are when we go to this slot, when we go to the next slot? How many possibilities when we go to the next slot right here? Well, we took 1 of the 6 out for the first slot, so there's only 5 possibilities here. And by the same logic when we go to the third slot, we've used up 2 of the slots-- 2 of the colors already, so there would only 4 possible colors left. And then for the last slot, we would've used up 3 of the colors, so there's only 3 possibilities left. So if we think about all of the possibilities, all of the permutations-- and permutations are when you think about all the possibilities and you do care about order; where you say that this is different than this-- this is a different permutation than this. So all of the different permutations here, when you pick 4 colors out of a possible of 6 colors, it's going to be 6 possibilities for the first 1, times 5 for the second bucket, times 4 for the third or the third bucket of the third position, times 3. So 6 times 5 is 30, times 4 is times 3. So 30 times 12. So this is 30 times 12, which is equal to their 360 possible 4-color codes.