Given:
Radius of orbit of X is R1=R
Radius of orbit of Y isR2=4R
Time period of orbit of X be T1
Time period of orbit of Y be T2
According to Kepler's Third Law,
Time period T∝R32
where R : Radius of the orbit.
⟹T2T1=R3/22R3/21
T2T1=(4RR)3/2=8
Abswer = 8