Frequency away from an oscillating body

[roberttk01]

This is the set up to the question that we got for homework:

Pit and Plank. A wooden plank is placed over a pit that is 5.00 m wide. A physics student stands in the middle of the plank and begins to jump up and down such that she jumps upward from the plank two times each second. The plank oscillates with a large amplitude, with maximum amplitude at its center.

Part A) Asks :
What is the speed of transverse waves on the plank? Answer: 20m/s

Part B)
At what rate does the student have to jump to produce large-amplitude oscillations if she is standing 1.25 m from the edge of the pit? (Note: The transverse standing waves of the plank have nodes at the two ends that rest on the ground on either side of the pit.)


-Part B is where I am having issues. I tried using the added length and the same speed, the offset length and same speed, and the original length minus the new length with the same speed and nothing has produced what it is looking for. Please help?

 

[haruspex]

Harmonics, perhaps?