Question

a source produces 15 crusts and 15 trough when the second crust is produce the 1st
is 2 cm away from source
calculate frequency,wavelength,speed of wave

Answer 1

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