The difference between simple and compound interests compounded annually on a certain sum of money for 2 years at 18% per annum is Rs 81. The sum is ________.

The formula for the difference (D) between compound interest and simple interest for 2 years is:
D=
100
2

P⋅R
2

​

Where:
D=Difference=Rs 81
P=Principal sum
R=Rate of interest per annum=18%

Substitute the given values into the formula:

81=
100
2

P⋅18
2

​

81=
10000
P⋅324
​

Solve for P:

P=
324
81×10000
​

Since 324=81×4:

P=
4
10000
​
=2500
Thus, the principal sum is Rs 2500.