1. ## 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...

2. ## Re: permutation and combination

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.

3. ## Re: permutation and combination

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)

4. ## Re: permutation and combination

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.