Question

on what sum of money does the difference between the simple interest and compound interest in 2 years at 5% per annum is 15

Answer 1

compound interest let be ci;
simple interest let be si
p let be 15; 
r let be 5%;
t let be 2 years

then,si=15*5*2/100 =3/2=1.5

ci=15(1+5/100)² - 15
   =15*105/100*105/100 - 15
   =165375/10000 - 15
   =1323/80 - 15
   =123/80 =1.54[approx]

different between ci & si=1.54 - 1.5 =0.04
so,the ans is 0.04

Answer 2

Let sum is x. Then,
C. I. = [x{(1+5/100)²-1} = 1025x/10000 = 41x/400;
S. I. = (x*5*2)/100 = x/10.
W. K. T., 41x/400 - x/10 = 15.
⇒x = 6000.
∴ The sum of money is Rs. 6000.