If a certain sum of money at 5% per annum simple interest amounts to ₹810 in 2
2
1
​
years, then the certain sum is:

Given:
Amount (A) = ₹810

Rate (R) = 5% per annum

Time (T) = 2
2
1
​
=2.5 years

The relation between Amount (A) and Principal (P) under simple interest is:

A=P+SI=P+
100
P×R×T
​
=P(1+
100
R×T
​
)
Substituting the values:

810=P(1+
100
5×2.5
​
)
810=P(1+
100
12.5
​
)
810=P(1+0.125)
810=1.125P
P=
1.125
810
​
=
1125
810000
​
=720
Therefore, the principal sum is ₹720.