Two springs of force constant 1000 N/m & 2000 N/m are stretched by same force. The ratio of their respective potential energies is

potential energy PE = 12KX2  ; K=force constant; x = displacement     restoring force =F =-KX                               ⇒FK=-X ;  substitute X value in PE equation, PE =12KF2K2 =12F2K for two springs, PE1 =12F2K1  PE2 =12F2K2  divide the above two equations, one by the other PE1 PE2=K2K1  ; substitute the given vales of K1,K2 PE1 PE2==20001000=21