Speed of a Wave: Solve for V in 4.6m and 7.4s

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The problem involves calculating the speed of a wave based on the distance between crests and the time taken for multiple crests to pass. The observer notes that five wave crests pass in 7.4 seconds, with a distance of 4.6 meters between successive crests. The initial calculation incorrectly assumes the period based on five crests instead of the four wavelengths between them. Correctly identifying that there are four wavelengths gives a period of 1.85 seconds. Using the corrected period, the speed of the wave is recalculated to be approximately 3.11 m/s.
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Homework Statement


A person fishing from a pier observes that five wave crests pass by in 7.4 s and estimates the distance between two successive crests to be 4.6 m. The timing starts with the first crest and ends with the fifth. What is the speed of the wave?



Homework Equations


f=1/T
v=λf

The Attempt at a Solution


I solved for the period first, so in 7.4 sec there was 5 waves. T=1.48
f=1/T=.6757
v=λf=4.6*(.6757)=3.11 m/s

I know my error has to do wit that sentence, "The timing starts with the first crest and ends with the fifth," but I'm not sure what I'm doing wrong.
 
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Draw a sketch of a wave and number the crests 1 through 5. How many wavelengths are contained between 1 and 5?
 
There are 4 wavelengths.
 
Good. So what should the period T actually be?
 

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