The simple interest on a certain sum of money at the rate of 5% p.a. for 8 years is Rs. 840.At what rate of interest the same amount of interest can be received on the same sum after 5 years ?  Options:
A) 6 %
B) 8 %
C) 9 %
D) 10 %
E) none

Rationale: "solution s . i . = rs . 840 , r = 5 % , t = 8 years . principal = rs . ( 800 x 840 / 5 x 8 ) = rs . 2100 . now , p = rs . 2100 , s . i . = rs . 840 , t = 5 years . ∴ rate = ( 100 x 840 / 2100 x 5 ) = 8 % . answer b"
Answer: B