If the amount obtained by Mahesh by investing Rs.1500 for 2.5 years at 8% p.a. is equal to the amount obtained by Suresh by investing a certain Sum for 2 years at 5% p.a. S.I., then the find the Sum invested by Suresh.

To solve this problem, we need to understand that the amount obtained by both Mahesh and Suresh is the same.

1. First, let's calculate the amount obtained by Mahesh. He invested Rs.1500 for 2.5 years at 8% per annum simple interest. The formula for simple interest is PRT/100, where P is the principal amount, R is the rate of interest, and T is the time in years.

   So, the interest obtained by Mahesh = (1500 * 8 * 2.5) / 100 = Rs. 300.

   The total amount obtained by Mahesh = Principal + Interest = 1500 + 300 = Rs. 1800.

2. Now, we know that Suresh also obtained the same amount, Rs. 1800, by investing a certain sum for 2 years at 5% per annum simple interest.

   Let's denote the sum invested by Suresh as 'S'.

   So, the total amount (Principal + Interest) = S + (S * 5 * 2) / 100 = 1800.

3. Solving the above equation for 'S', we get:

   S + 0.1S = 1800, 1.1S = 1800, S = 1800 / 1.1 = Rs. 1636.36.

So, the sum invested by Suresh is approximately Rs. 1636.36.