Which parts of the curve in the figure shown represent compression and rarefaction for a longitudinal wave?

A narrow sound pulse (for example, a short pip by a whistle) is sent across a medium. If the pulse rate is $1$ after every $20s$ (that is the whistle is blown for a split of second after every $20s$)is the frequency of the note produced by the whistle equal to $\frac{1}{20}$ or $0.05Hz$ ?

Waves are generated on a water surface. Calculate the phase difference between two points $A$ and $B$, when, $A$ and $B$ lie on the same wavefront at a distance of $2\lambda$ between them, 

Waves are generated on a water surface. Calculate the phase difference between two points $A$ and $B$, when $A$ and $B$ lie on successive crests separated by $1 \mathrm{m}$

Waves are generated on a water surface. Calculate the phase difference between two points A and B, when 

A and B lie on successive troughs separated by $1.5 \mathrm{m}$