The ancient Romans used Roman numerals. We still use them today. Here are some examples: $$\rm I=1$$, $$\rm II=2$$, $$\rm V=5$$, $$\rm IX=9$$, $$\rm X=10$$, $$\rm XI=11$$, $$\rm XX=20$$. This year($2022$) we celebrate Math Kangaroo number $$\rm XX$$. What year was Math Kangaroo number $$\rm XV$$? 
Overseas Competition->Knowledge Point->Combinatorics->Logical Reasoning->Reasoning by Conditions->Complex Reasoning 
According to the pattern, $$\rm XX$$ is $$20$$, and $$\rm XV$$ is $$15$$.  So, Math Kangaroo number $$15$$ was $$5$$ less than real year, which was $$2017$$. 