Sometimes this type of sum are given by the teaches in which logarithm cannot be used at grade or class 8 level. Teacher want to puzzle students so that more students join to their private coaching. You need not to worry . We are here to help the needy students. Let first reed the Sum .

This sum will be shown by two method. first one is preferable for students of class 8 to class 10

Question:- A sum of money becomes double in 3 years at a compound rate of interest. Howmany years It will be its 8 times at the same rate. Find the compound rate of interest and the time in which it becomes its 8 times.

1st method

solution: Let ‘x’ becomes 2x in 3 years. We know that : A=P(1+r/100)^{t}A=Amount

P= Principal

r=compound rate of interest

t=time

∵ 2x=x (1+r/100)^{3}

∴ 2 = (1+ r/100)^{3}

Now 8x=x(1+r/100)^{t}

8 = (1+r/100)^{t}

Or 2 ^{3}= (1+ r/100)^{t}

or [(1+ r/100)^{3}]^{3}=(1+r/100)^{t}

or (1+ r/100)^{3×3}=(1+r/100)^{t}

(1+ r/100)^{9}=(1+r/100)^{t}

Now the base (1+ r/100) is same at both side.

Therefore, t = 9 years. ANS.

2nd method

∵ 2x=x (1+r/100)^{3}

∴ 1+ r/100=(2)^{(1/3)}

∴ 1+r/100=1.2599

∴ r=26

Now 8x=x(1+26/100)^t

or 8=(1.26)^t

or (1.26)^9=(1.26)^t

Therefore, t=9 years. ANS.

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