The problem was discussed on Mr Kantaro's Youtube Channel in Japanese.
Thanks for your post. I never knew that Simplify function has been so useful. Mathematica could make it easy to solve entrance exams.
When I was a student, I solved problems with paper and pencil. Those were the good old days.
-- you have earned Featured Contributor Badge
Your exceptional post has been selected for our editorial column Staff Picks http://wolfr.am/StaffPicks and Your Profile is now distinguished by a Featured Contributor Badge and is displayed on the Featured Contributor Board. Thank you, keep it coming, and consider contributing to the The Notebook Archive!