At what simple rate of interest a sum is double in 8 years ?
Answer: (b) 12.5%
Let the rate be R% and principal be Rs. X
Sum is double in 8 years,
Then, X = X x R x 8/100
R = 100/8
R = 25/2
R = 12.5%
Then, Rate of simple interest 12.5% a sum is doubled in 8 years.
Hence, the correct answer is option (b) 12.5%