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**1. 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

**2. Homework Equations**

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

v=f[tex]\lambda[/tex] or v=[tex]\lambda[/tex]/t

**3. The Attempt at a Solution**

I have no idea how to solve this problem