How many distinct arrangements of four letters (without repeats) from the set {a, b, c, d, e} are possible? 15?
1 answer:
From the pool of 5 letters, we're picking 4. We can arrange the 4 we picked in 
ways.
But there are 5 ways of *not* picking the remaining letter. To account for the arrangements containing the letter not picked, we multiply this total by 5.
Put another way, suppose we drew a, b, c, and d, but not e. We have 24 possible arrangements for these 4 letters, but for any distinct arrangement there are 5 ways of replacing any letter with e.
So the number of arrangements we can actually make is
.
But there are 5 ways of *not* picking the remaining letter. To account for the arrangements containing the letter not picked, we multiply this total by 5.
Put another way, suppose we drew a, b, c, and d, but not e. We have 24 possible arrangements for these 4 letters, but for any distinct arrangement there are 5 ways of replacing any letter with e.
So the number of arrangements we can actually make is
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P=7+7+12+12P=14+24P=38
See the attachment for diagram
See the attachment for diagram
The purpose of the grid is to help you record possible and impossible combinations. The repeat of the headings 1st, 2nd, 3rd, 4th helps you compare to both shirt color and name.
Each given statement has certain implications. You are expected to be able to determine as many of those as you can, and mark the grid accordingly.
The first statement says two girls slide down before two other girls. This means the first two girls listed cannot be 3rd or 4th in line, and the last two girls listed cannot be 1st or 2nd in line. It also means that none of the named girls is wearing the blue shirt (so Madeline must be wearing it). These implications are marked in the attachment with X and a circled 1, to indicate to you that the first statement was where we got the X. The X is used to signify an impossible combination.
The second statement tells you the purple shirt will not be 3rd or 4th in line, since at least two girls are after it. It tells you that Alexis is not wearing purple or yellow (so must be wearing white), and it tells you that Alexis and the yellow shirt are not first in line. (We alread knew that about Alexis.)
The third statement lets you finish the problem. We just found that Alexis is wearing the white shirt, so it tells you Madeline is 2nd in line and Alexis is 3rd. (Alexis is 3rd or 4th, but so is Lauren. The only way Madeline can be next to Alexis is for Madeline to be 2nd and Alexis 3rd.) The check marks labeled with a circled 3 are the conclusions we drew from statement 3.
Since the yellow shirt is not first, Lauren must be wearing it 4th in line.
The final conclusion is ...
- Hanna, purple
- Madeline, blue
- Alexis, white
- Lauren, yellow
Once you have identified a combination, it can help to mark off the others in that row and column (of the group of 4 rows and columns) as being impossible. This can narrow the choices for the others. I didn't do that here because it makes the diagram busier and the sequence of marks less clear.
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It is a good idea to check your final answer with the original problem statements to make sure the answer is consistent with them.
