Simple interest
 At simple interest, a sum doubles after 20 years. The rate of interest per annum is ?

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Let sum = P, then SI = P and Time = 20 years
∴ Required rate = (100 x SI)/(P x T) = (100 x P) / (P x 20) = 5% per annumCorrect Option: A
Let sum = P, then SI = P and Time = 20 years
∴ Required rate = (100 x SI)/(P x T) = (100 x P) / (P x 20) = 5% per annum
 The simple interest at R% for R years will be Rs. R on a sum of ?

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Sum = (100 x SI) / (R x T)
= (100 x R) / (R x R)Correct Option: C
Sum = (100 x SI) / (R x T)
= (100 x R) / (R x R)
= Rs. 100/R
 A sum of money at simple interest amounts to Rs. 2240 in 2 years and Rs. 2600 in 5 years. The sum is ?

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S.I for 3 years = Rs. (2600  2240) = Rs . 360
S.I for 2 years = Rs. (360/3 x 2) = Rs. 240Correct Option: B
S.I for 3 years = Rs. (2600  2240) = Rs . 360
S.I for 2 years = Rs. (360/3 x 2) = Rs. 240
∴ Required sum = Rs . (2240  240) = Rs. 2000
 At the certain rate of simple interest, a certain sum doubles itself in 10 years. It will treble itself in ?

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Let principal = P. Then, S. I = P. and Time = 10 years
∴ Required time = [(n  1) x t] / (m  1)Correct Option: B
Let principal = P. Then, S. I = P. and Time = 10 years
∴ Required time = [(n  1) x t] / (m  1)
= [(3  1) x 10] / (2  1)
= 20 years
 A sum of money put at simple interest trebles itself in 15 years. The rate per cent per annum is ?

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Let principal = P
Then, SI = 2P & Time = 15 years
∴ Required rate = (100 x SI) / (P x T) = (100 x 2P) / (P x 15)Correct Option: A
Let principal = P
Then, SI = 2P & Time = 15 years
∴ Required rate = (100 x SI) / (P x T) = (100 x 2P) / (P x 15) = 200/15 = 13^{1}/_{3} per annum