One complete wave passes a putrefying pile of poached pickerel in 0.50 seconds. the piled pickerel protrude 20 cm along the beach. calculate its wavelength

If the wave passes the putrefying pile of a poached pickerel in 0.5 seconds, then the wave has a period of 0.5s. 

But we say that if F=1/T,
where F⇒frequency,
          T⇒period.

F=1/0.5
  = 2Hz

velocity of the wave = distance/time taken
                                = (20÷100)/0.5
                                = 0.2÷0.5
                                = 2/5

Now form the wave equation,
V=λF 
Where V ⇒ velocity of the wave.
             λ⇒wavelength of the wave.
       
So, λ=V/F

    λ = 2/5 ÷ 2
       =2/5 ×1/2
       = 2/10
       =0.2 m

The wavelength in cm is, 0.2×100 = 20 cm.