Question 885944
One fast shortcut is the <a href="http://en.wikipedia.org/wiki/Rule_of_72">rule of 72</a>. Divide 72 by 10 (ignore the percent) to get 72/10 = 7.2



So it takes roughly 7.2 years



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Let's use the formula to get a more accurate answer:



A = P*e^(r*t)


20000 = 10000*e^(0.10*t)


20000/10000 = e^(0.10*t)


2 = e^(0.10*t)


ln(2) = ln(e^(0.10*t))


ln(2) = 0.10*t*ln(e)


ln(2) = 0.10*t*1


ln(2) = 0.10*t


0.10*t = ln(2) 


t = ln(2)/0.10


t = 6.93147180559946


t = 7 ... round to the nearest whole number



So it takes approximately 7 years (which is reasonably close to 7.2)



The final answer is 7 since they want you to use the formula and round to the nearest year. However, it's always good to know a fast shortcut such as the <a href="http://en.wikipedia.org/wiki/Rule_of_72">rule of 72</a>.