tolong jawab soal no. 8 beserta cara kerjanya...... tolong bantuannya yaaah

dik : Ra = 2 cm ; ωa = 20 rpm ; ωb = 13 rpm
dit : Rb = ...??
jawab :
ωa.Ra = ωb.Rb
20.(2) = 13.Rb
Rb = 3,07 cm ≈ 3 cm (jawab : C)