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Aptitude question and answer on Simple interest



1
In how many years will a sum of money double itself at 4% per annum ?
A.    8 years
B.    16 years
C.    12 years
D.    25 years

Answer : D.  25 years

Explanation:
Let  Sum = x 
Then  S.I = x
\(\dot{..}\) Time = \({100 \times S.I \over Sum \times Rate}\)
             =\(\bigg({100 \times x \over x \times 4} \bigg)\)years
             = 25 years

2
A person borrowed Rs. 1500 at the rate of 5% per annum S.I. what amount will he pay to clear the debt after 4 years ?
A.    Rs. 2000
B.    Rs. 1880
C.    Rs. 1800
D.    Rs. 1600

Answer : C.  Rs. 1800

Explanation:
Amount = Rs. \(\Bigg( 1500+{1500 \times5 \times4 \over100}\Bigg)\)=Rs. 1800

3
Rajnish took a loan for 7 years at the rate of 6% p.a. S.I. If the total paid was Rs. 2100, the principal was
A.    2%
B.    5%
C.    8%
D.    10%

Answer : A.  2%

Explanation:
Rate=\( \Bigg( {100 \times 300 \over5000 \times3} \Bigg)\%=2\%\)

4
At a certain rate of simple interest, a certain sum doubles itself in 10 years. If will triple itself in 
A.    12 years
B.    15 years
C.    20 years
D.    30 years

Answer : C.  20 years

Explanation:
Let     sum  = x.
Then   S.I.  = x and time = 10 years
\(\dot{..}\) Rate = \(\Bigg( {100 \times x \over x \times 10} \Bigg) \%=10\%\)
Now , Sum = x, S.I.=2x , Rate = 10 %
\(\dot{..}\) Time = \(\Bigg( {100 \times 2x \over x \times 10} \Bigg) \)years
                = 20 years

5
The simple interest on Rs. 18200 from March 9, 2001 to May 21, 2001 at of \(7{1 \over 2}\) rate percent will be
A.    Rs. 290
B.    Rs. 273
C.    Rs. 288
D.    Rs. 225

Answer : B.   Rs. 273

Explanation:
March + April + May = 22+30+21
                              = 73 days
                              =\({73 \over 365}\) year
                              = \({1 \over 5}\) year
\(\dot{..}\)              Interest  = Rs. \(\Bigg(18200 \times{1 \over5} \times {15 \over 2 \times 100} \Bigg)\)
                              =Rs. 273

6
A certain sum of money lent out at S.I. amounts to Rs. 6900 in 3 years and Rs. 7500 in 5 years. The sum lent is
A.    Rs. 4000
B.    Rs. 4500
C.    Rs. 5000
D.    Rs. 6000

Answer : A.  Rs. 4000

Explanation:
S.I. for 2 years = Rs (7500-6900)
                      = Rs. 600
S.I for 3 years = Rs. \(\bigg( {600 \over 2} \times 3 \bigg)\)
                      =Rs. 900
\(\dot{..}\)     Principal = Rs. (6900-900)
                     = Rs. 6000



7
A Sum of money triples itself in 15 years 6 months. in how many years would it double itself ?
A.    6 years 3 months
B.    7 years 9 months
C.    8 years 3 months
D.    9 years 6 months

Answer : B.   7 years 9 months

Explanation:
Let sum = x.
Then 
S.I.=2x and Time = \(31 \over 2\)years
\(\dot{..}\) Rate = \(\Bigg({100 \times 2x \over x} \times{2 \over31} \Bigg)\%\)
           =\(\bigg({400 \over 31} \bigg)\%\)
Now,       Sum=x,    S.I.=x
and      Rate = \(\bigg({400 \over 31} \bigg)\%\)
\(\dot{..}\) Time = \(\Bigg({100 \times x \over x} \times{31 \over 400} \Bigg)\)year
            =7 years 9 months.

8
For how many years should Rs. 1200 be invested at 10% p.a. in order to earn the same simple interest as is earned by investing Rs. 1800 at 12% p.a. for 5 years ?
A.    9
B.    10
C.    12
D.    11

Answer : A.  9

Explanation:
S.I required = Rs. \(\Bigg({1800 \times 12 \times 5 \over 100} \Bigg)\)
                 = Rs. 1080
         Time = \(\Bigg({1800 \times 1080 \over 1200 \times10} \Bigg)\)years
                 = 9 years