Let the number of prizes of Rs. 100 be \( x \), and the number of prizes of Rs. 50 be \( 200 - x \).
Step 1: The total amount spent on the prizes is: \[ 100x + 50(200 - x) = 12000 \]
Step 2: Simplify the equation: \[ 100x + 10000 - 50x = 12000 \] \[ 50x + 10000 = 12000 \] \[ 50x = 2000 \]
Step 3: Solve for \( x \): \[ x = \frac{2000}{50} = 40 \] Thus, the number of prizes of Rs. 100 is 40.