There is a 1.2 m distance between a crest and an adjacent trough in a series of waves on the surface of a lake. In 30s, 35 crests pass a buoy anchored in the lake. What is the speed of these waves?

Answer 1

The wavelength is the crest-to-crest distance, which would be 2.4 m.

Multiply that by frequency to get the wave speed.

The frequency is 35/30 = 1.17 Hz

Answer 2

1.2x2x34/30

Distance between two crest points is 1.2x2
Distance between 35 crest points is 1.2x2x34.
Note a crest is a point. One may not measure the time duration of a crest point.

I've just noted that waves and crests are both used in such problems. The total distance is commonly given by multiplying the number, or 35 in this case. To me, this is really confusing!