Exponential growth and compound interest

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alexandria
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Homework Statement


upload_2016-5-23_19-58-57.png


Homework Equations


A = P(1+i)^n

The Attempt at a Solution


Here is my attempted solution, can someone please verify if my method is correct!
Thanks in advance![/B]
upload_2016-5-23_19-58-38.png
 
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This is a good example of the "rule of 72", which is a rule of thumb used by financial people. Basically it says that the number of years it takes for an investment to double is equal to 72 divided by the annual percentage rate. In this case 72/4 = 18, quite close to your more exact solution.
 
ok thanks for the help!