其它
On $2021$ December $31$\textsuperscript{st}, Lucas, Jeremy, and Irene visited their grandpa together. Then Lucas visited him every $4$ days, Jeremy visited him every $5$ days, and Irene visited him every $6$ days. In the first three months of $2022$, how many days could the grandpa be visited by at least one person? 
Overseas Competition->Knowledge Point->Counting Modules->Inclusion-Exclusion Principle
In $31+28+31=90$ days, they could visit the grandpa in $[90\div4]+[90\div5]+[90\div6]-[90\div20]-[90\div12]-[90\div30]+[90\div60]=42$ days. 