151. A certain sum doubles in 7 years at simple interest. The same sum under the same interest rate will become 4 times in how many years.
     

1. 1. 14
      

   
   
   

   2. 28
      

   
   
   

   3. 21
      

   
   
   

   4. 10

   
   
   

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1. View Hint View Answer Discuss in Forum

   According to question ,
   Case I,
   Here , T = 7 years
   Simple Interest = Principal
   

   Rate =	SI × 100
   	Principal × Time

   
   

   Rate =	100	% per annum
   	7

   
   Case II,
   Here , R = ( 100 / 7 )%
   Simple Interest = 3 × Principal
   

   Time =	SI × 100
   	Principal × Rate

   
   

   Correct Option: C

   According to question ,
   Case I,
   Here , T = 7 years
   Simple Interest = Principal
   

   Rate =	SI × 100
   	Principal × Time

   
   

   Rate =	100	% per annum
   	7

   
   Case II,
   Here , R = ( 100 / 7 )%
   Simple Interest = 3 × Principal
   

   Time =	SI × 100
   	Principal × Rate

   
   

   Time =	3 × 100
   	100
   	7

   
   Time = 3 × 7 = 21 years

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152. A sum amounts to double in 8 years by simple interest. Then the rate of simple interest per annumis
     

1. 1. 10%
      

   
   
   

   2. 12.5%
      

   
   
   

   3. 15%
      

   
   
   

   4. 20%

   
   
   

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1. View Hint View Answer Discuss in Forum

   Let Principal = Rs. p
   Amount = Rs. 2p
   ∴ Simple Interest = Amount - Principal
   Simple Interest = 2p – p = Rs. p
   

   ∴ Rate =	SI × 100
   	Principal × Time

   
   

   Rate =	p × 100	=	25
   	p × 8		2

   
   Rate = 12.5 % per annum
   Second method to solve this question :
   Here , n = 2
   

   R =	(n - 1)	× 100
   	T

   
   

   Correct Option: B

   Let Principal = Rs. p
   Amount = Rs. 2p
   ∴ Simple Interest = Amount - Principal
   Simple Interest = 2p – p = Rs. p
   

   ∴ Rate =	SI × 100
   	Principal × Time

   
   

   Rate =	p × 100	=	25
   	p × 8		2

   
   Rate = 12.5 % per annum
   Second method to solve this question :
   Here , n = 2
   

   R =	(n - 1)	× 100
   	T

   
   

   R =	(2 - 1)	× 100%
   	8

   
   R = 12.5%

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153. In how much time, will a sum of money become double of itself at 15% per annum simple interest?
     

1. 1. 6	1	years
      	4

   
   
   

   2. 6	1	years
      	2

   
   
   

   3. 6	1	years
      	3

   
   
   

   4. 6	2	years
      	3

   
   
   

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1. View Hint View Answer Discuss in Forum

   If the principal be p, the amount = 2p
   ∴ SI = p
   

   ∴ Time =	SI × 100
   	Principal × Rate

   
   

   Time =	p × 100	=	20	= 6	2	years.
   	p × 15		3		3

   
   Second method to solve this question :
   

   T =	(n - 1)	× 100
   	R

   
   

   Correct Option: D

   If the principal be p, the amount = 2p
   ∴ SI = p
   

   ∴ Time =	SI × 100
   	Principal × Rate

   
   

   Time =	p × 100	=	20	= 6	2	years.
   	p × 15		3		3

   
   Second method to solve this question :
   

   T =	(n - 1)	× 100
   	R

   
   

   T =	(2 - 1)	× 100
   	15

   
   

   T =	100	year
   	15

   
   

   T =	20	year
   	3

   
   

   T = 6	2	year
   	3

   
   

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154. At a certain rate of simple interest, a certain sum of money becomes double of itself in 10 years. It will become treble of itself in
     

1. 1. 15 years
      

   
   
   

   2. 18 years
      

   
   
   

   3. 20 years
      

   
   
   

   4. 30 years

   
   
   

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1. View Hint View Answer Discuss in Forum

   If principal be p , interest = p and rate = r% p.a. then
   

   Rate =	SI × 100
   	Principal × Time

   
   

   Rate =	p × 100	= 10%
   	p × 10

   
   Now, Principal = p, interest = 2p
   

   Then, time =	SI × 100
   	Principal × Rate

   
   

   time =	2p × 100	= 20 year
   	p × 10

   
   Second method to solve this question :
   

   R =	(2 - 1)	× 100%
   	10

   
   R = 10%
   

   Correct Option: C

   If principal be p , interest = p and rate = r% p.a. then
   

   Rate =	SI × 100
   	Principal × Time

   
   

   Rate =	p × 100	= 10%
   	p × 10

   
   Now, Principal = p, interest = 2p
   

   Then, time =	SI × 100
   	Principal × Rate

   
   

   time =	2p × 100	= 20 year
   	p × 10

   
   Second method to solve this question :
   

   R =	(2 - 1)	× 100%
   	10

   
   R = 10%
   

   T =	(n - 1)	× 100 years.
   	R

   
   

   T =	3 - 1	× 100 years.
   	100

   
   T = 20 years.

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155. In how many years will a sum of money double itself at 6	2	%
     	3

     simple interest per annum ?
     

1. 1. 24 years
      

   
   
   

   2. 20 years
      

   
   
   

   3. 16 years
      

   
   
   

   4. 12 years

   
   
   

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1. View Hint View Answer Discuss in Forum

   Let principal is P.
   As we know that ,
   

   Time =	SI × 100
   	Principal × Rate

   
   

   Time =	P × 100	= 16 years
   	P × (25 ÷ 4)

   
   Second method to solve this question :
   

   T =	(n - 1)	years
   	R%

   
   

   Correct Option: C

   Let principal is P.
   As we know that ,
   

   Time =	SI × 100
   	Principal × Rate

   
   

   Time =	P × 100	= 16 years
   	P × (25 ÷ 4)

   
   Second method to solve this question :
   

   T =	(n - 1)	years
   	R%

   
   

   T =	(2 - 1)	× 100 years
   	25÷4

   
   T = 16 years.

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