Question

a certain amount of money doubles itself in 8 years. In how much time it becomes 4 times of itself at the same rate of simple interest.

Answer 1

Here,
P=principal
I=simple interest
N=no. of years
R=rate of interest
doubles means p=I
so
I=PNR/100
P=PNR/100
NR=100
R=100/N
R=100/8
R=25/2
NOW
IT IS 4 TIMES MEANS....I=3P
I=PNR/100
3P=PNR/100
300=NR
300=N25/2
600/25=N
N=24
SO .....ANS IS
24 YEARS

Answer 2

let the principle be rs. 100
therefore , amount = rs. 100 *2
= Rs. 200
therefore ,simple interest = Rs. 100
time = 8 years
rate = (100 * 100)/(100 * 8) 
= 25/2
= 12.5 %


let the principle be Rs. 100
amount = RS100 *4
= Rs. 400
simple interest = Rs.400 - Rs. 100
= Rs. 300
rate = 12.5 %
time = (300*100)/(100*12.5)
= (300*100*10)/(100*125)
= 24 years.