Aptitude - Simple Interest - Discussion

Thanks for your explanation.

@Kannan.

As we say here that we have found out the interest for 3 years i.e, 117. That's why we subtracted 117 from 815.

If, we calculate the interest for 4 years..the amount will be, (39*4)= 156, then we should subtract 156 from 854, the will be same as above.

P = (A2 * T1 - A1 * T2)/(T1 - T2).

@All.

Principle = the amt u borrowed
Interest = the small additional amount you pay for the amount you borrowed

First, let me explain to you The question.

In the question,

A sum of money at simple interest amounts to Rs. 815 in 3 years means the Principle amt together wit the interest paid for 3 yrs is Rs. 815 and similarly Principle amt together wit the interest paid for 4 yrs is Rs. 854.

Therefore, if we subtract (854-815) we get the simple interest for 1 year as Rs. 39.
Then, we calculate simple interest for 3 yrs as (39*3)= 117.
Next step, we subtract (815 - 117) to get Principle amount for a yr as Rs. 698

Brief explanation for last step:

815 = Principle amt + int for 3 yrs ---> equation 1.
117 = int for 3 yrs ---> equation 2.

Now subtract both equations, we get the principle amount.

Why we have subtracted 117 from 815 why not from 854?

S.I. for 1 year = Rs. (854 - 815) = Rs. 39.
S.I. for 3 years = Rs.(39 x 3) = Rs. 117.
Principal = Rs. (815 - 117) = Rs. 698.

And if you want to calculate from 854
then,
SI for 4 year =Rs(39*4)=156,
Principal =Rs (854 - 156)= 698.

Let x=principal interest.
x+3*SI = 815.
x+4*Si = 854.

SI for 1 Year=854-815=39,
x+3*39=815,
x=815-117=698.

Thank you @Gayatri.

I am not getting it, can anyone explain me clearly?

Given:

First amount = Rs. 815.
Second amount=Rs. 854.

Diff of amount = second amount - first amount.
= 854-815.
= 39.
Now, for 3 years;
You will have to multiply.
=39*3.
=117.
So now principal is;
= 815-117,
p= 698.