Aptitude

1

A sum of money placed at compound interest doubles itself in 4 years. In how many years will it amount to 8 times?

a

9 years

b

8 years

c

27 years

d

12 years

Let,
Principal = Rs. 100.
Amount = Rs. 200.
Rate = r%
Time = 4 years.
Now,
A = P*[1+ (r/100)]^n;
200 = 100*[1+(r/100)]^4;
2 = [1+(r/100)]^4; ........... (i)
If sum become 8 times in the time n years,
then,
8 = (1+(r/100))^n;
2^3 = (1+(r/100))^n; ........(ii)
Using eqn (i) in (ii), we get;
([1+(r/100)]^4)^3 = (1+(r/100))^n;
[1+(r/100)]^12 = (1+(r/100))^n;
Thus, n = 12 years.

2

The difference between simple and compound interest for the fourth year is Rs. 7280 at 20% p.a. What is the principal sum?

a

78653

b

67000

c

50000

d

55000

= ((Pr) /100) *[(1 +(r /100))^(n-1) -1]
→ 7280 = ((P*20)/100) *[(1.2)^3-1]