Question

A sum amounts to ₹1326 in 6 years at 5% p.a. simple interest. In what time will this sum double itself at same rate of interest ??

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ans it correctly
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Answer 1

Let the sum = P

Amount = Principal + Interest

I = (PRT)/100

A = P + I
1326 = P + (Px5x6)/100
1326 = P + 0.3P
1326 = 1.3P
P = 1326/1.3
   = 1020

For the sum to double it will come to 1020x2 = 2040
    
2040 = 1020 + (1020x5xT)/100
2040-1020 = 51T
1020 = 51T
T = 1020/51
T = 20

The sum will double itself in 20 years

Answer 2

Let the principal be rupees 100
T=6 years
R=5 per cent p.a
S.I. = 100×6×5/100=30
A=P+I =100 +30=130
If the Ais 130 then P=rupees 100
If the A is 1 then P=100/130
If the A is 1326 then P=100/130×1326=rupees 1020
A.T.Q
A=1020 ×2= 2040
therefore P=1020
A=2040
I = A-P
2040 -1020= 1020
T= 1020×100/1020×5= 20 years