# permutation and combination

• July 29th 2013, 08:12 AM
Trefoil2727
permutation and combination
4 alphabets are selected from the alphabets in the word LEMBAGA. Find the number of selection of 4 different alphabets which can be made so that it
a. contains only 1 alphabet A
b. contains 2 alphabets A

I always get confuse with this type of question, so...
• July 29th 2013, 08:23 AM
Plato
Re: permutation and combination
Quote:

Originally Posted by Trefoil2727
4 alphabets are selected from the alphabets in the word LEMBAGA. Find the number of selection of 4 different alphabets which can be made so that it
a. contains only 1 alphabet A
b. contains 2 alphabets A

The answers are $\\a)~\binom{5}{3}\\b)~\binom{5}{2}$

Now if you ever want more help here, you must explain why those are correct.
• July 30th 2013, 03:39 AM
Trefoil2727
Re: permutation and combination
Quote:

Originally Posted by Plato
The answers are $\\a)~\binom{5}{3}\\b)~\binom{5}{2}$

Now if you ever want more help here, you must explain why those are correct.

em, I thought
a) 4 alphabets are selected from 7 alphabets, of the 7, 2 are alphabets A. So, 7-2=5, then I thought I have to use 5C3 x 2C1( because there are 2 alphabets A)
• July 30th 2013, 03:48 AM
Plato
Re: permutation and combination
Quote:

Originally Posted by Trefoil2727
em, I thought
a) 4 alphabets are selected from 7 alphabets, of the 7, 2 are alphabets A. So, 7-2=5, then I thought I have to use 5C3 x 2C1( because there are 2 alphabets A)

But the two A's are identical. We cannot tell one from the other. So there is only one way to select an A.