A certain sum amounts to Rs.640 in 4 years and Rs.760 in 6 years. Find the rate % per annum?  Choices:
A) 9%  B) 5%  C) 3%  D) 1%  E) 8%
4 --- 640   6 --- 760   --------------   2 --- 120   N = 1 I = 60 R = ?   P = 640 - 240= 
<gadget id="calculator">640 - 240</gadget>
<output>400</output>
400   60= (400*1*R)/100   R = 15%   Answer: 8%
<result>E</result>
A certain sum amounts to Rs.640 in 4 years and Rs.760 in 6 years. Find the rate % per annum?