Are there any more mathematical journals or websites with the "problems and solutions"? such as , the American Mathematical Monthly and Crux Mathematicorum
Problems columns I found (in 2009) useful to challenge undergraduates. Some of them may no longer be current. But even those may be interesting to consult past issues in your school library!
Mathematics Magazine https://www.maa.org/press/periodicals/mathematics-magazine
Mathematical Mayhem https://www.facebook.com/pg/Mathematical-Mayhem-1253467234689081/
Nieuw Archief http://www.nieuwarchief.nl/homeitems.php?onderwerp=6