Aptitude

Aptitude Questions and Answers

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

correct answer d

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

correct answer c

Difference between CI and SI for nth year,
= `((Pr) /100) *[(1 +(r /100))^(n-1) -1]`
→ `7280 = ((P*20)/100) *[(1.2)^3-1]`
→ P = 50000.

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