Find magnitude of Potential Energy?

Question

A catapult throws a stone of mass 0.10kg with a velocity og 30 ms-1.If 25% of the potential entery of elastic band is wasted during trasmission? (i want answer to be 60J)

Answer 1

Potential energy here by consevation of energy is given by :

            P.E  =  Kinetic energy  +   Energy consumed by  

                                                          non consevative forces

Thus if P.E  =  x

            x  =  1/2(0.1) {30}^2  +  (25/100)x

     =>   (3/4) x  =  45

     =>       x  =  {(45) 4 }/3   =   (15 )4  =  60J

Answer 2

KE at launch = (1/2)*0.10*(30^2) =45 J

PE before launch = 45/0.75

45/0.75 = 60 J "answer"

check:

PE lost =0.25*60 = 15 J