Concept:The money doubles every 3 years, so to become 8 times (23), it takes 3 doubling periods.Explanation:Let the principal be P.After 3 years, amount =2P, so P(1+r)3=2P implies (1+r)3=2.We need n such that P(1+r)n=8P, i.e., (1+r)n=8.Since 8=23, we have (1+r)n=[(1+r)3]3=(1+r)9.Thus n=9 years.Answer:9 years