请教一题

ninaluan

if money is invested at r percent interest, com-pounded annually, the amount of the ivestment will double in approximately 70/r years. if pat's parents invested $5000in a long term bond that pays 8 percent interest, compunded annually, what will be approximate total amount of the investment 18 years later, when pat is ready for college?
A. $20,000
B. $15,000
C. $12,000
D. 10,000
e.9,000
答案为A请问如何解得？谢谢！

kathy72

投资5000刀，70/8＝8.5年后double,为10000刀，再8.5年后变成20000刀。题目问的是approximate total amount。