Venn diagrams and principle of inclusion/exclusion practice.
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The 3 Ghostbusters have 12 tools they use.
Dr. Peter Venkman uses 7 tools.  Dr. Raymond Stanz uses 6 tools.

Pete and Ray share 4 tools.  Pete and Egon share 4 tools.
Ray and Dr. Egon Spengler share 3 tools.  All three Ghostbusters
share 2 tools.

1. How many Ghosbusters tools does Egon use?

2. How many tools do Ray and Egon share, that Pete never uses?

3. How many tools do Pete and Egon share, that are never used by Ray?

4. How many tools of the Ghostbuster's 12 tools are exclusively used
   by only a single Ghostbuster?