According to work-energy theorem, KE of stone = Elastic potential energy of rubber band. Now, for rubber band; Young's modulus =5×108N∕m2 Length, L=2×10−2m Change of length, ∆L=2×10−2m Area of cross-section, A=5×10−6m2 Elastic potential energy of rubber band =
1
2
×Y×(strain)2×volume =
1
2
×Y×(
∆L
L
)2×A×L =
1
2
×5×108×(
2×10−2
2×10−2
)2×5×10−6×2×10−2 =25J This energy is given to stone ( mass =20g); ⇒KE(stone)=E.P.E(Rubber‌band) ⇒