simple harmonc motion
Let equations of two SHMs be x=Asin(wt) and x=Asin(wt+a), where A,w, be common amplitude and angular frequency, and a be phase difference between two SHMs.
distance between two particles at any time t =| A(sin(wt+a)-sin(wt))|
max value is when |cos((2wt+a)/2)|=1
max distance=2A|sin(a/2)|=20cm=A; or, |sin(a/2)|=1/2
or, sin(a/2)=1/2 or -1/2,
a/2=pi/6 rad or -pi/6 rad
a = pi/3 rad or -pi/3 rad.
therefore SHMs have a phase difference of pi/3.