Step 1: Calculate the price at \( x = 450 \)
The price is given by:
\[
p = 450 - \frac{x}{2} = 450 - \frac{450}{2} = 225.
\]
Step 2: Compute the rebate
The original price is Rs. 350. The rebate is:
\[
\text{Rebate} = 350 - 225 = 125 \, \text{(Rs. per calculator)}.
\]
Step 3: Final result
The rebate required to maximise the revenue is Rs. 125 per calculator.