Question

what should be the interest rate for an amount to become 5 times of self at simple interest in 100 years ?

Answer 1

let the principal amount be P time= t=100 interest rate=r we have 5P=P(1+100r) 5=(1+100r) r=4/100=4% rate of interest is 4%

Answer 2

Take P=x, Then amount is 5x(5 times of x). Time =100 years(time is given)
we know that SI= Amount-principal
Therefore, SI=5x-x = 4x
now the formula to find rate is :- 
R=SI x 100/P x T = (4x x 100/x.100)%=4% 
 verification:-
Let p= Rs.1 
 R=4%  and Time=100 years
SI=Rs (1 x 4 x 100)/100 = Rs 4
 Amount = P + SI = Rs 1 + Rs 4 = Rs 5, which is 5 times of 1.
          Hence verified too.