Finding wavelength from counting crest

[aryan]

Homework Statement

[/B]
A stone tossed into a body of water creates a disturbance at the point of impact that lasts for Dt = 4.0s. Measurements indicates that the wave speed is v = 25 cm/s. (a) Over what distance on the surface of water does the wave group extend? (b) An observer counts 12 wave crests in the group.Use the classical uncertainty relationship to estimate the precision with which the wavelength could be determined.

Homework Equations

delta(x) * delta (k) >= PI[/B]

The Attempt at a Solution

the group distance is calculated by multiplying wave speed*the time impact (b) there are 12 crest over the distance of 100 cm hence the wavelength will be 100/12. I am not sure about this is there any mistake in this?[/B]

 

[Simon Bridge]

Welcome to PF;
The only thing I can think of is that 12 crests is not always the same as 12 wavelengths ... sketch a wave with 12 crests and count the wavelengths.