Among the natural numbers $1-75$: How many are divisible by $3$ or $5$? 
Overseas Competition->Knowledge Point->Counting Modules->Inclusion-Exclusion Principle->Inclusion-Exclusion Principle for Two Sets
All natural numbers divisible by $3$ in the range of $1-75$ are：$75\div3=25$．  All natural numbers divisible by $5$ in the range of $1-75$ are：$75\div5=15$．  The natural numbers divisible by 3 and 5 that are divisible by $15$：$75\div15=5$．  The natural numbers divisible by $3$ or $5$ are：$25+15-5=35$． 