Trapped in a circular box, shot at 1000 frames per second .
On this video, you can observe that the walking droplet has a period which is the double of the associated waves period : the waves oscillates twice between each impact of the droplet. This is the so-called (2,1) mode
It is also the double of the forcing period : you might count the numbers of frames between each impact of the droplet on the bath surface : approximately 27. Hence a period of 27/1000 sec (Camera shoots at 1000 frames per second) Hence a frequency of 37 HZ. Which is half the forcing frequency: 75 HZ.