Given Information:
- Principal amount (P) = ?
- Amount after 3 years (A) = Rs. 1199/-
- Rate of interest (R) = 12 1/2 % per annum
- Time (T) = 3 years

Formula for Simple Interest:
The formula to calculate simple interest is:
\[I = \frac{P \cdot R \cdot T}{100}\]
Where,
I = Simple Interest
P = Principal amount
R = Rate of interest
T = Time (in years)

Calculating the Principal Amount:
We can rearrange the formula for simple interest to find the principal amount:
\[P = \frac{I \cdot 100}{R \cdot T}\]

Substituting the Given Values:
Given that the amount after 3 years is Rs. 1199/-, we can calculate the simple interest using the formula:
\[I = A - P\]
Substituting the values:
\[I = 1199 - P\]

Now, substituting the values of I, R, and T into the formula for P:
\[P = \frac{(1199 - P) \cdot 100}{12.5 \cdot 3}\]

Solving the Equation:
Simplifying the equation:
\[P = \frac{119900 - 100P}{37.5}\]
Multiplying both sides by 37.5 to eliminate the denominator:
\[37.5P = 119900 - 100P\]
Combining like terms:
\[137.5P = 119900\]
Dividing both sides by 137.5 to isolate P:
\[P = \frac{119900}{137.5}\]

Calculating the Principal Amount:
Evaluating the expression:
\[P = \frac{119900}{137.5} = Rs. 872\]

Therefore, the principal amount is Rs. 872.

Conclusion:
The principal amount is Rs. 872, and it will amount to Rs. 1199/- in 3 years at 12 1/2 % per annum in simple interest.