Aptitude - Simple Interest - Discussion

If certain principal amounts to A1 in T1 year and to A2 in T2 year ,then the sum is given by = A2 .T1--A1.T2
_____________
T2--T1
Now use the formula
= 854 * 3 -- 815 * 4
________________
4--3
= 2562--3260
____________
1
= 698.

Let principal = x,

Principal + simple interest = total sum (after any number of years),

For 3 years,

X + x * r * 3 = 815 ----> Eqution 1,

For 4 years,

X + x * r * 4 = 854 ----> Equation 2,

Now there are 2 unknown and 2 equations solve for x and we get.

X = 698.

I will explain in a simple way.

We have A = P + I.

815 = P + I -----> Eq 1.

854 = P + I -----> Eq 2.

Subtract Eq 1 and 2.

Simple interest I = 815 - P - 854 + P

= 39 for 1year.

Then take for 3 years or 4 years.

You will get the same answer.

For 3 years:

39 * 3 = 117.

Or For 4 years:

39 * 4 = 156.

Then use formula A = P + I.

For 3 years:

815 = P + 117,

P = 815 - 117,

= 698.

For 4 years:

854 = P + 156,

P = 854 - 155,

= 698.

Take for 3 or 4 years answer will be same. I think the explanation is very useful.

Let the sum is Rs. X and rate of interest is y%.

We know A = P + SI where SI = (P * R * T) /100 Therefore, A = P + (P * R * T) /100.

Case 1 : A = 815, P = x, T = 3 years, R = y%.
A = x + (x * y * 3) /100->1

Case 2 : A = 854, P = x, T = 4 years, R = y%.

A = x + (x * y * 4) /100 ->2

Subtracting equation (1) from equation (2), we get x * y = 3900.

Now by putting this value of x * y = 3900 in any one of above two equation, we'll get sum = x = 698 Answer.

Thanks @Vikram and @Gayatri.

@Appu.

Your explanation is good, Thank you.

Crystal clear explanation, Thanks @Gayathri.

Here you can do it by another S. I 1 yr = 39, in 4 years =156, & principle=854 - 156 = 698.

@Gayathri nice explanation.

@Gayathri.

Thank you for explaining the solution clearly.