Heres the exercise:

9. (CAROUSEL) Four children are rotated on a carousel and

their speed on the ground is v. Kids sit at the same time as the squares ABCD, at the same distance from the rotation axis and throw each other ball. When Angelo, at the top of A, throws the ball directly at the C point opposite Carl, the ball actually reaches Berta sitting at the top B. At what speed Angelo ball was thrown?

Note: the cos x = x equation x ≈ 0.739rad.

However, if x is the angle in degrees, then the corresponding equation cos x = πx◦ is 42.3◦. Created: Andre Sääsk

In that website exercise 9 is one picture, too, if its helping:

https://www.teaduskool.ut.ee/sites/d...ded_noorem.pdf