[strstr]

Bunch of problems like this are in the book "Problem-Solving Strategies" by Arthur Engel [1], I believe even including this particular one. Fun book.

[1]https://www.google.com/books/edition/Problem_Solving_Strateg...

[taeric]

Or anything with Martin Gardner's name on it. All worth reading.

[romaththrow592]

This book is a classic one for kids participating in math competitions in Romania. I once had to solve a variation of this problem at a math contest when I was in 6th grade.

[rodneyzeng]

Yeah, got this book and the problem was also a classical one.