Math Is Fun Forum
  Discussion about math, puzzles, games and fun.   Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫ • π ƒ -¹ ² ³ °

You are not logged in.

#1 2006-02-26 17:20:46

ganesh
Moderator
Registered: 2005-06-28
Posts: 12,919

Permutations and Combinations

PC # 1

There are 7 men and 3 ladies. Find the number of ways in which a committee of 6 persons can be formed if the committee is to have atleast 2 ladies.


Character is who you are when no one is looking.

Offline

#2 2006-02-26 17:56:40

krassi_holmz
Real Member
Registered: 2005-12-02
Posts: 1,908

Re: Permutations and Combinations

first, I thank you ganesh for making this new themes.
(But problems and solutions is best, too)


IPBLE:  Increasing Performance By Lowering Expectations.

Offline

#3 2006-02-26 18:04:32

krassi_holmz
Real Member
Registered: 2005-12-02
Posts: 1,908

Re: Permutations and Combinations

So if there are 2 ladies in the com, we have
(7)C(6-2) combinations. Multiply this by the number of 2-ladies:3C2
And simular, if we have 3l:
NUMBER=(7C4)(3C2)+(7C5)(3C3)=21
Is this the answer?


IPBLE:  Increasing Performance By Lowering Expectations.

Offline

#4 2006-02-27 02:16:04

ganesh
Moderator
Registered: 2005-06-28
Posts: 12,919

Re: Permutations and Combinations

Two ladies must be in the committee.
If two ladies are chosen, the number of combinations is 3C2.
The remaining 4 members can be chosen from the 7 men and the number of combinations is 7C4.
Hence,  the number of ways in which two ladies can form part of the committee is 3C2*7C4.
If three ladies are chosen, the number of combinations is 3C3.
The remaining 3 members can be chosen from the remaining 7 men and the number of combinations is 7C3.
Hence, the number of ways in which three ladies can form part of the committee is 3C3*7C3.
The total number of combinations is 3C2*7C4 + 3C3*7C3.
That is, 105 + 35 = 140.


Character is who you are when no one is looking.

Offline

#5 2006-02-27 02:40:04

krassi_holmz
Real Member
Registered: 2005-12-02
Posts: 1,908

Re: Permutations and Combinations

One simple muistake may change the answer badly:
(7C4)(3C2)+(7C5)(3C3)=21
(7C4)(3C2)+(7C3)(3C3)=140


IPBLE:  Increasing Performance By Lowering Expectations.

Offline

#6 2006-02-28 16:16:05

ganesh
Moderator
Registered: 2005-06-28
Posts: 12,919

Re: Permutations and Combinations

PC # 2

There are 6 books on Economics, 3 on Mathematics and 2 on Accountancy. In how many ways can they be arranged on a shelf if the books of the same subject are always to be together?


Character is who you are when no one is looking.

Offline

#7 2006-02-28 17:51:59

krassi_holmz
Real Member
Registered: 2005-12-02
Posts: 1,908

Re: Permutations and Combinations

They must be tigether so we have 3 subjects that must be arranged:
AEM
AME
EAM
EMA
MAE
MEA


IPBLE:  Increasing Performance By Lowering Expectations.

Offline

#8 2006-02-28 19:46:51

ganesh
Moderator
Registered: 2005-06-28
Posts: 12,919

Re: Permutations and Combinations

Read the question fully......and.....
try_again.gif


Character is who you are when no one is looking.

Offline

#9 2006-03-01 18:05:26

ganesh
Moderator
Registered: 2005-06-28
Posts: 12,919

Re: Permutations and Combinations

PC # 3

How many numbers are there between 100 and 1000 such that atleast one of their digits is 6?


Character is who you are when no one is looking.

Offline

#10 2006-03-02 04:39:11

mathsyperson
Moderator
Registered: 2005-06-22
Posts: 4,900

Re: Permutations and Combinations

PC # 3

All of the numbers from 600 to 699 have a 6. This leaves 800 numbers between 100 and 999.
Of those, 1/10 will have 6 as their tens digit. 800/10 = 80, so that leaves 720 more.
Of those, 1/10 will have 6 as their units digit. 720/10 = 72, and adding this to 80 and 100 will give the answer.

100+80+72 = 252.


Why did the vector cross the road?
It wanted to be normal.

Offline

#11 2006-03-02 16:33:52

ganesh
Moderator
Registered: 2005-06-28
Posts: 12,919

Re: Permutations and Combinations

goodwork.gif


Character is who you are when no one is looking.

Offline

#12 2006-03-09 03:40:19

ganesh
Moderator
Registered: 2005-06-28
Posts: 12,919

Re: Permutations and Combinations

PC # 4

Bus number plates contain three distinct English alphabets followed by four digits with the first digit not zero. How many different number plates can be formed?


Character is who you are when no one is looking.

Offline

#13 2006-03-11 03:49:57

ganesh
Moderator
Registered: 2005-06-28
Posts: 12,919

Re: Permutations and Combinations

PC # 5

The figures 4, 5, 6,7, and 8 are written in every possible order. How many of the numbers so formed will be greater than 56,000?


Character is who you are when no one is looking.

Offline

#14 2006-05-12 07:41:07

John E. Franklin
Member
Registered: 2005-08-29
Posts: 3,552

Re: Permutations and Combinations

Guess to PC#4
    (26) × (25) × (24) × (9) × (10) × (10) × (10)
(Algebra never should have used x's, it sure makes multiplying more confusing.)


igloo myrtilles fourmis

Offline

#15 2006-05-12 16:47:12

ganesh
Moderator
Registered: 2005-06-28
Posts: 12,919

Re: Permutations and Combinations

You're correct, John! Well done smile
You could use . instead of x for multiplication sign!


Character is who you are when no one is looking.

Offline

#16 2007-09-16 23:22:02

landof+
Member
Registered: 2007-03-24
Posts: 131

Re: Permutations and Combinations

ganesh wrote:

PC # 5

The figures 4, 5, 6,7, and 8 are written in every possible order. How many of the numbers so formed will be greater than 56,000?

First, find ALL the combinations

Since the formula is

where r is
(the number of positions)-1

so now we see: combinations which don't accept 4's at the start and 54's.

and we have:

Done smile

Last edited by landof+ (2007-09-16 23:23:23)


I shall be on leave until I say so...

Offline

#17 2007-09-17 11:09:37

John E. Franklin
Member
Registered: 2005-08-29
Posts: 3,552

Re: Permutations and Combinations


Woops I forgot some 57xyz, 58xyz, so add 12 to my answer sorry.

Last edited by John E. Franklin (2007-09-17 11:11:15)


igloo myrtilles fourmis

Offline

#18 2007-09-17 23:24:43

landof+
Member
Registered: 2007-03-24
Posts: 131

Re: Permutations and Combinations

Which is 90, same I suppose big_smile big_smile big_smile


I shall be on leave until I say so...

Offline

#19 2008-07-22 17:07:15

Sudeep
Member
Registered: 2008-07-21
Posts: 20

Re: Permutations and Combinations

krassi_holmz wrote:

They must be tigether so we have 3 subjects that must be arranged:
AEM
AME
EAM
EMA
MAE
MEA

the answer should be !6 * !3 *!2 * !3

the books can be arranged among themselves as
!6  Economics,
!3 on Mathematics 
!2 on Accountancy

and !3 among themselves

Offline

#20 2008-08-20 19:58:10

Sudeep
Member
Registered: 2008-07-21
Posts: 20

Re: Permutations and Combinations

There are 3 boys and 3 girls. In how many ways can they be arranged so that each boy has at least one girl by his side?

Offline

#21 2008-08-21 19:24:18

Sudeep
Member
Registered: 2008-07-21
Posts: 20

Re: Permutations and Combinations

There are 10 boxes numbered 1, 2, 3, …10. Each box is to be filled up either with a black or a white ball in such a way that at least 1 box contains a black ball and the boxes containing black balls are consecutively numbered. The total number of ways in which this can be done is..

Offline

#22 2008-10-12 00:09:37

All_Is_Number
Member
Registered: 2006-07-10
Posts: 258

Re: Permutations and Combinations

ganesh wrote:

Two ladies must be in the committee.
If two ladies are chosen, the number of combinations is 3C2.
The remaining 4 members can be chosen from the 7 men and the number of combinations is 7C4.
Hence,  the number of ways in which two ladies can form part of the committee is 3C2*7C4.
If three ladies are chosen, the number of combinations is 3C3.
The remaining 3 members can be chosen from the remaining 7 men and the number of combinations is 7C3.
Hence, the number of ways in which three ladies can form part of the committee is 3C3*7C3.
The total number of combinations is 3C2*7C4 + 3C3*7C3.
That is, 105 + 35 = 140.

There are three ladies from which to choose the two slots designated for ladies. After those two slots are filled, there are eight unchosen members from which to randomly assign the remaining four committee seats. Why isn't the answer nCr(3,2)*nCr(8,4)=3*70=210? What am I missing? If I'm counting 70 possibilities twice, which possibilities am I double counting?

Edit to add: I'm reasonably confident the method I proposed is wrong, and Ganesh's solution is indeed correct. I'm interested in understanding why the method I proposed is wrong.

Last edited by All_Is_Number (2008-10-13 18:32:34)


You can shear a sheep many times but skin him only once.

Offline

#23 2008-10-12 00:19:00

All_Is_Number
Member
Registered: 2006-07-10
Posts: 258

Re: Permutations and Combinations

Sudeep wrote:

There are 10 boxes numbered 1, 2, 3, …10. Each box is to be filled up either with a black or a white ball in such a way that at least 1 box contains a black ball and the boxes containing black balls are consecutively numbered. The total number of ways in which this can be done is..


You can shear a sheep many times but skin him only once.

Offline

#24 2010-10-19 08:04:54

bobbym
Administrator
From: Bumpkinland
Registered: 2009-04-12
Posts: 81,417

Re: Permutations and Combinations

This problem is old but...

The correct answer is 55, by direct count.

You can compute it by:

pointing_finger.png


In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

Offline

Board footer

Powered by FluxBB