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...

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 $\displaystyle \\a)~\binom{5}{3}\\b)~\binom{5}{2}$

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

Re: permutation and combination

Quote:

Originally Posted by

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

**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)

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.