36.
A sum of money becomes Rs. 6690 after three years and Rs. 10035 after six years on compound interest. The sum is -
• A.
Rs. 4400
• C.
Rs. 4460
• B.
Rs. 4445
• D.
Rs. 4520
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Explanation :

Let the principal be P, then

 P 1 + R 3 = 6690 ...(i) 100

 and P 1 + R 6 = 10,035 ...(ii) 100

Now, Dividing (ii) by (i), we get

1 + R 3 = 10035 = 3
100 6690 2

P x 3  = 6690
2

P = 6690 x 2  = Rs. 4460
3

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37.
A bank offers 10% interest rate compounded annually. A person deposits Rs. 10,000 every year in his account. If he does not withdraw any amount, then how much balance will his account show after four years?
• A.
Rs. 51051
• C.
Rs. 36410
• B.
Rs. 45095
• D.
Rs. 51000
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Explanation :

 Rs 10000 after 4 years become

= 10000
 1 + 10 100
4

 = 10000 x 11 x 11 x 11 x 11 = 14641 10 10 10 10

 Rs 10000 after 3 years become

 = 10000 x 11 x 11 x 11 = 13310 10 10 10

 Rs 10000 after 2 years become

 = 10000 x 11 x 11 = 12100 10 10

 Rs 10000 after 1 year become Rs 11000

 Total amount after 4 years = Rs 51051.

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38.
A sum of money at compound interest amounts to Rs. 578.40 in 2 years and to Rs. 614.55 in 3 years. The rate of interest per annum is
• A.
4%
•  C. 6 1 % 4
• B.
5%
•  D. 8 1 % 3
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Explanation :

 Rate = (614.55-578.40) x 100 578.40

 = 3615 = 6.25% 578.40

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39.
What annual payment will discharge a debt of Rs. 1025 due in 2 years at the rate of 5% compound interest?
• A.
Rs. 550
• C.
Rs. 560
• B.
Rs. 551.25
• D.
Rs. 560.75
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Explanation :

Let each instalment be Rs. x. Then,

 x + x = 1025 1 + 5 1 + 5 2 100 100

20x + 400x  = 1025
21 441

820x = 1025 x 441   x =   1025 x 441    = 551.25
820

So, value of each instalment = Rs. 551.25.

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40.
A sum of money placed at compound interest doubles itself in 4 years. In how many years will it amount to 8 times?
• A.
6 years
• C.
12 years
• E.
None of these
• B.
8 years
• D.
10 years
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Explanation :

 Amount = Principal 1 + r n 100

 In first case 2 = 1 1 + r 4 100

and in second case

 8 = 1 1 + r n 100

 or, (2)3 = 1 + r n 100

or,
 1 + r 4 100
3 =
 1 + r n 100

or,
 1 + r 12 100
=
 1 + r n 100

 n = 12

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