Calculation of Simple Interest:

Given,
Principal amount (P) = 12000
Total Amount (A) = 16500
Time (T) = 2 1/2 years = 2.5 years

Formula for Simple Interest:
Simple Interest (SI) = P x R x T / 100

Where,
P = Principal amount
R = Rate of interest
T = Time in years

To find the rate of interest, we can use the following formula:
Rate of Interest (R) = (SI x 100) / (P x T)

Substituting the given values, we get:
SI = A - P = 16500 - 12000 = 4500
T = 2.5 years

Calculating the rate of interest using the above formula, we get:
R = (4500 x 100) / (12000 x 2.5)
R = 15%

Therefore, the rate percent per annum simple interest is 15%.

Explanation:
Simple interest is a type of interest that is calculated on the principal amount only. It is a fixed percentage of the principal amount that is applied to the loan or investment on an annual basis. In this problem, we are given the principal amount, the total amount, and the time period for which the interest is to be calculated. Using the formula for simple interest, we can calculate the amount of interest that has been earned over the given time period. Once we have calculated the interest, we can use the formula for rate of interest to find the annual percentage rate at which the interest has been earned. In this case, the rate of interest is 15%.