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It is not given clearly in how many seconds the 15 crests and 15 troughs are produced.  Let us assume that  they are produced in one (1) second.

   Then frequency  f  is 15 Hertz or 15 / second, a it is the number of crests produced in a second.
 The distance between successive crests is the wavelength =>
     λ = wave length = 2 cm

       speed of wave = f * λ = 30 cm /second

1 5 1