investment is pretty big. if the investment grows at a rate of 20% per year, then it takes less than 5 years for the value of the investment to double. that means it takes less than 15 years for the investment to double three times, which is a factor of . to grow by a factor of 10, it really should not take more than 15 years, at the roughest estimate. this leaves choices a (5 years), b (10 years), and c (12.6 years). clearly 5 years is too short. after all, an investment that grows at 20% per year cannot grow 1000% in five years. this leaves choice b 