# Combinatorics / Inclusion/Exclusion Question

• Oct 12th 2012, 12:09 AM
floorplay
Combinatorics / Inclusion/Exclusion Question
Hi everyone, first time posting on this forum and was hoping if someone would be willing to explain how to do the below question. Any help appreciated, thanks.

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• Oct 14th 2012, 09:37 AM
Plato
Re: Combinatorics / Inclusion/Exclusion Question
Quote:

Originally Posted by floorplay
Hi everyone, first time posting on this forum and was hoping if someone would be willing to explain how to do the below question. Any help appreciated, thanks.

The question is to find the number of solutions to $\sum\limits_{k = 1}^5 {u_k } = 24,\quad 4 \le u_1 \;\& \;u_2 < 7$
Let $S_k$ be the event that $u_1=k$.
Let $T_k$ be the event that $u_2=k$.
Let $\|S_k\|$ be the number of solutions in $S_k$.

The answer to the posed question is $\sum\limits_{k = 4}^6 {\sum\limits_{j = 4}^6 {\left\| {S_k \cap T_j } \right\|} }$,
where $\left\| {S_k \cap T_j } \right\|=\binom{26-k-j}{2}$.