Concept:The doubling time formula for compound interest: A=P(1+r)t.Explanation:Let the principal be P and the amount after 2 years be 2P.Using the formula A=P(1+r)2, we get 2P=P(1+r)2.Cancel P: (1+r)2=2.Take square root: 1+r=2​=1.4142135.Thus r=1.4142135−1=0.4142135.Convert to percentage: r×100%=41.42135%.Answer:41.42135%