1. Good post? |

## Combinatorics - Counting

I was browsing the web to find intriguing questions for quant section especially in Combinatorics.
Bumped on to the following question.

How many ternary strings of length 4 have exactly one 1?
[Courtesy : http://web.eecs.utk.edu/~booth/311-0...natorics.html]

My answer for this question was,
_,_,_,_ 4 spaces needs to be filled with 0,1 and 2
out of which one of them needs to be 1. there are 4 ways in which one of them could be 1
and in all other instances there would be choices of 2 numbers in filling 3 of these blanks
4*(23)

32 strings have exactly one 1 in it.

According to the site where i got this question from though, the answer is different.
[3*(23)]

Can anyone clarify what's the logical fallacy in my answer? or elaborate more on why it could be wrong?

2. Good post? |

## Re: Combinatorics - Counting

Originally Posted by pranavanmaru

How many ternary strings of length 4 have exactly one 1?
Way out of scope for the GRE.

Cheers,
Brent - Greenlight Test Prep