Question

a sum of money become 8 by 5 of itself in 5 years at a certain rate of simple interest find the rate of interest (answer will be 12%).

Answer 1

Let the principal be P. 
A/Q A = 8/5P 
Given T=5years
WKT, SI = A-P 
= 8P/5 -P 
SI =3P/5
Again ,
SI =PRT/100
3P/5=PRT/100
3/5=5R/100
R=3*20/5
R=3*4
R=12

The required rate of interest is 12%

Answer 2

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Refer to the attachment!