Destructive Interference with a Pebble

[luvinlife]

Homework Statement

A pebble is dropped in a lake; and it produces ripples with a frequency of .25 Hz. When should a second pebble be dropped at the same place to produce destructive interference

Homework Equations

I'm not even sure if these apply, but they are the only formulas I know.

v=f$$\lambda$$ or v=$$\lambda$$/t

The Attempt at a Solution

I have no idea how to solve this problem

 

[CrazyIvan]

What will the second pebble's wave look like? How do these two waves "add up"?

In what way do you want them to "add up" in order to get destructive interference

 

[luvinlife]

I get it!
wow.. definitely over thought that one.
Thank you for the thought provoking questions.