Question

certain sum of money double itself in 8 years calculate the rate of interest

Answer 1

let the Sum be RS.100
amount after specified time=100*2=200
simple interest=A-P=200-100=100
time given=8yrs
R=SI*100/P*T
R=100*100/100*8
=100/8
=12.5%
RATE=12.5% P.A

Answer 2

Let the principal be $100
time period = 8yrs
let the rate of interest be 'r' % per annum
Amount = 2*$100
             = $200
Therefore, Interest = Amount - Principal
                               = $200 - $100
                               = $100
I = P*100/r*t
$100 = $100*100/8r
=> 800r = 10000
=>       r = 10000/800
=>       r = 12.5

Hence, required rate of interest is 12.5% per annum.