# Thread: Hi guys!Need help with combinatorics problem!

1. Good post? |

## Hi guys!Need help with combinatorics problem!

How many two-element subsets of (1,2,3,4) are there that do not contain the pair of elements 2 and 4?
a) 1
b)2
c)4
d)5
e)6

How did you solve? Explanation please!

2. Good post? |

## Re: Hi guys!Need help with combinatorics problem!

You can do this by taking 4C2 - 1.

Or, this is a pretty easy problem to just go through the combinations. Given the choices, you have at most 6 combinations to identify.

Here are the combos:

1,2
1,3
1,4
2,3
2,4
3,4

Look six combinations, but don't count (2,4). So 5 different subsets.

3. Good post? |

## Re: Hi guys!Need help with combinatorics problem!

Thanks!

Originally Posted by YaSvoboden
You can do this by taking 4C2 - 1.

Or, this is a pretty easy problem to just go through the combinations. Given the choices, you have at most 6 combinations to identify.

Here are the combos:

1,2
1,3
1,4
2,3
2,4
3,4

Look six combinations, but don't count (2,4). So 5 different subsets.