If money is invested at r percent interest, compounded annually, the amount of investment will double in approximately 70/r years. If Pat's parents invested $ 7000 in a long term bond that pays 8 percent interest, compounded annually, what will be the approximate total amount of investment 18 years later, when Pat is ready for college?	Choose the correct choice from the following answers.
A) $ 28000  B) $ 15000  C) $ 12000  D) $ 10000  E) $ 9000

Rationale: "since investment doubles in 70 / r years then for r = 8 it ' ll double in 70 / 8 = ~ 9 years ( we are not asked about the exact amount so such an approximation will do ) . thus in 18 years investment will double twice and become ( $ 7,000 * 2 ) * 2 = $ 28,000 ( after 9 years investment will become $ 7,000 * 2 = $ 14,000 and in another 9 years it ' ll become $ 14,000 * 2 = $ 28,000 ) . answer : a ."
Answer: A