Joe invested a certain sum of money in a simple interest bond whose value grew to $310 at the end of 3 years and to $410 at the end of another 5 years. What was the rate of interest in which he invested his sum? Choose the most appropriate option.
A) 6%
B) 7%
C) 8%
D) 9%
E) 10%
In 5 years, the value grew $100, so the simple interest was $20 per year.   In 3 years, the total interest was 3*$20 = $60   The principal is $310 - $60 3* 20= 
<gadget id="calculator">3* 20</gadget>
<output>60</output>
$60   The principal is $310 - $60 = 250.   The interest rate is $20/$250 310 -  60= 
<gadget id="calculator">310 -  60</gadget>
<output>250</output>
250.   The interest rate is $20/$250= 8%   The answer is 8%.
<result>C</result>
Joe invested a certain sum of money in a simple interest bond whose value grew to $310 at the end of 3 years and to $410 at the end of another 5 years. What was the rate of interest in which he invested his sum?