Venn Diagram explained from AoPS : learn the two set and three set examples
Principle of Inclusion-Exclusion from AoPS: again, two set and three set examples
Video on Venn Diagrams with Two categories from AoPS
Video on Venn Diagrams with Three categories from AoPS
Word problems on Venn diagrams medium level (instant feedback)
Word problems on Venn diagrams medium hard level (instant feedback)
The common error for most students (especially boys, oh oh) is arithmetic error.
Make sure you line the category with specific number given right and check your math.
Chicken, rabbit algebra speed test : aim for less than 5 minutes. No calculator, please !!
Motivation, Not IQ, Matters Most For Learning New Math Skills from Time Magazine
Power of Ten Video