If money is invested at r percent interest, compounded annually, the amount of the investment will double in approximately 24/r years. If Pat's parents invested $6,000 in a long-term bond that pays 8 percent interest, compounded annually, what will be the approximate total amount of the investment 18 years later, when Pat is ready for college?  Choices
A) $20000  B) $15000  C) $12000  D) $10000  E) $30000
Since investment doubles in 24/r years, then for r=8 it'll double in   24/8= 
<gadget id="calculator">  24/8</gadget>
<output>3</output>
~3 years (we are not asked about the exact amount so such an approximation will do). Thus after 18 years investment will become $5,000*6=$30,000 .   Answer: $30000.
<result>E</result>
If money is invested at r percent interest, compounded annually, the amount of the investment will double in approximately 24/r years. If Pat's parents invested $6,000 in a long-term bond that pays 8 percent interest, compounded annually, what will be the approximate total amount of the investment 18 years later, when Pat is ready for college?