RE: Math mini-contest problem for Day 3 on D.Buzz for May 2021 😎

I think I don't fully get it...

     /|B
   /  |
A/____|C

sin(A) = BC/AB; BC=15 according to Fang, but AB still unknown.
cos(A) = AC/AB; we could substitute AB here, but AC still unknown
according to Manga: sin(A) < cos(A), so the angle at A is < pi/4.
Without knowing this information doesn't allow to calculate cos(A), we only know that it can only be between 0.707 and 1 from A < pi/4
I must be missing something...