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In this question, since it is said one by one, why can't I use the formula (5C1 x 4C1 ) x (3C1 x 2C1) ?
CrackverbalGMAT
There are 5 adults and 3 children in a meeting. If 2 adults and 2 children are randomly chosen to give a speech one by one, how many different arrangements of speeches are possible?

No of ways you can select 2 adults out of 5 = 5C2 = \(\frac{5*4}{2!}\)= 10

No of ways you can select 2 children out of 3 = 3C2 = \(\frac{3*2}{2!} \)= 3

So the total no of ways you can select 2 adults and 2 children in the meeting = 5C2 * 3C2

For every 4 selected people, no of different orders they can give the speech = 4!

For Example:
if A1,A2,C1,C2 are selected out of 4 adults A1,A2,A3,A4 and 3 children C1,C2,C3.

No of different orders where A1,A2,C1,C2 can give the speech is 4! = 24 as the speech is given one by one.

Writing down the possible speech orders below.

1. A1,A2,C1,C2
2. A1,A2,C2,C1
3. A1, C1,A2,C2
4. A1, C1,C2,A2
5. A1,C2,A2,C1
6. A1,C2,C1,A2
..
..
..
..
..
..
..
24. C2,C1,A2,A1

So the total no of different arrangements of speeches possible for all combinations = 5C2 * 3C2 * 4! = 10* 3 * 24 = 720

Option E is the correct answer.

Thanks,
Clifin J Francis,
GMAT SME
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Did not get the underlying logic for the formula you used, but even though it is one by one you don't know the order of their speeches whether it was A1 A2 C1 C2, or C1 A2 A1 C2 - hence once you select 2 people from each group, you have to calculate the possible arrangements in which they can give their speech one by one.

Combinatorics made easy!

Hope it helps.
RakeshKiran
In this question, since it is said one by one, why can't I use the formula (5C1 x 4C1 ) x (3C1 x 2C1) ?
CrackverbalGMAT
There are 5 adults and 3 children in a meeting. If 2 adults and 2 children are randomly chosen to give a speech one by one, how many different arrangements of speeches are possible?

No of ways you can select 2 adults out of 5 = 5C2 = \(\frac{5*4}{2!}\)= 10

No of ways you can select 2 children out of 3 = 3C2 = \(\frac{3*2}{2!} \)= 3

So the total no of ways you can select 2 adults and 2 children in the meeting = 5C2 * 3C2

For every 4 selected people, no of different orders they can give the speech = 4!

For Example:
if A1,A2,C1,C2 are selected out of 4 adults A1,A2,A3,A4 and 3 children C1,C2,C3.

No of different orders where A1,A2,C1,C2 can give the speech is 4! = 24 as the speech is given one by one.

Writing down the possible speech orders below.

1. A1,A2,C1,C2
2. A1,A2,C2,C1
3. A1, C1,A2,C2
4. A1, C1,C2,A2
5. A1,C2,A2,C1
6. A1,C2,C1,A2
..
..
..
..
..
..
..
24. C2,C1,A2,A1

So the total no of different arrangements of speeches possible for all combinations = 5C2 * 3C2 * 4! = 10* 3 * 24 = 720

Option E is the correct answer.

Thanks,
Clifin J Francis,
GMAT SME
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Thanks Krunaal for the reply. I will just elaborate what I was saying. There are two part to the solution: 1. Selection of Adults and children. 2. Ordering them for speeches.

For your solution:

1. Selection of 2 adults and 2 children
Number of ways = 5C2 x 3C2 --- since you have used combination formula and is selecting two adults at same time, the selection for (A1, A2 ) and (A2, A1) is counted as same.

2. Ordering them for speeches.
Since we know here order matters, you multiply the answer with 4! (because while selection you have considered (A1, A2) same as (A2, A1)).

For my solution:

1. Selection of 2 adults and 2 children
Since they have said one by one
number of ways to select first adult = 5C1
number of ways to select second adult (from remaining four) = 4C1. (Notice here I'm taking (A1, A2) and (A2, A1) as different)
There for number of ways = (5C1 x 4C1 ) x (3C1 x 2C1)

2. Ordering them for speeches
Since I have already counted the order as different I have to multiply by ( 4! / 2! x 2! ). This is because there are 2 children and 2 adults in the four selection which are identical. ( which is already counted during selection ).

This is my inference. Please correct me if I'm wrong. The confusion arise from the question part saying "one by one".
Or is it like that choosing two adults at a time from 5 adults and choosing two adults one by one from 5 adults is the same always.

Krunaal
Did not get the underlying logic for the formula you used, but even though it is one by one you don't know the order of their speeches whether it was A1 A2 C1 C2, or C1 A2 A1 C2 - hence once you select 2 people from each group, you have to calculate the possible arrangements in which they can give their speech one by one.

Combinatorics made easy!

Hope it helps.
RakeshKiran
In this question, since it is said one by one, why can't I use the formula (5C1 x 4C1 ) x (3C1 x 2C1) ?
CrackverbalGMAT
There are 5 adults and 3 children in a meeting. If 2 adults and 2 children are randomly chosen to give a speech one by one, how many different arrangements of speeches are possible?

No of ways you can select 2 adults out of 5 = 5C2 = \(\frac{5*4}{2!}\)= 10

No of ways you can select 2 children out of 3 = 3C2 = \(\frac{3*2}{2!} \)= 3

So the total no of ways you can select 2 adults and 2 children in the meeting = 5C2 * 3C2

For every 4 selected people, no of different orders they can give the speech = 4!

For Example:
if A1,A2,C1,C2 are selected out of 4 adults A1,A2,A3,A4 and 3 children C1,C2,C3.

No of different orders where A1,A2,C1,C2 can give the speech is 4! = 24 as the speech is given one by one.

Writing down the possible speech orders below.

1. A1,A2,C1,C2
2. A1,A2,C2,C1
3. A1, C1,A2,C2
4. A1, C1,C2,A2
5. A1,C2,A2,C1
6. A1,C2,C1,A2
..
..
..
..
..
..
..
24. C2,C1,A2,A1

So the total no of different arrangements of speeches possible for all combinations = 5C2 * 3C2 * 4! = 10* 3 * 24 = 720

Option E is the correct answer.

Thanks,
Clifin J Francis,
GMAT SME
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RakeshKiran
Thanks Krunaal for the reply. I will just elaborate what I was saying. There are two part to the solution: 1. Selection of Adults and children. 2. Ordering them for speeches.

For your solution:

1. Selection of 2 adults and 2 children
Number of ways = 5C2 x 3C2 --- since you have used combination formula and is selecting two adults at same time, the selection for (A1, A2 ) and (A2, A1) is counted as same.

2. Ordering them for speeches.
Since we know here order matters, you multiply the answer with 4! (because while selection you have considered (A1, A2) same as (A2, A1)).

For my solution:

1. Selection of 2 adults and 2 children
Since they have said one by one
number of ways to select first adult = 5C1
number of ways to select second adult (from remaining four) = 4C1. (Notice here I'm taking (A1, A2) and (A2, A1) as different)
There for number of ways = (5C1 x 4C1 ) x (3C1 x 2C1)

2. Ordering them for speeches
Since I have already counted the order as different I have to multiply by ( 4! / 2! x 2! ). This is because there are 2 children and 2 adults in the four selection which are identical. ( which is already counted during selection ).

This is my inference. Please correct me if I'm wrong. The confusion arise from the question part saying "one by one".
Or is it like that choosing two adults at a time from 5 adults and choosing two adults one by one from 5 adults is the same always.

Krunaal
Did not get the underlying logic for the formula you used, but even though it is one by one you don't know the order of their speeches whether it was A1 A2 C1 C2, or C1 A2 A1 C2 - hence once you select 2 people from each group, you have to calculate the possible arrangements in which they can give their speech one by one.

Combinatorics made easy!

Hope it helps.

RakeshKiran
In this question, since it is said one by one, why can't I use the formula (5C1 x 4C1 ) x (3C1 x 2C1) ?
Yes, you will have to multiply by \(\frac{4!}{2!2!}\) for your solution and you will get the correct answer. Choosing two adults at a time from 5 adults and choosing two adults one by one from 5 adults is not the same.
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Cant we just do 5C2 * 3C2. This selects the 4 people. Multiply by 4! for giving speech.

So 10*3*4!
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