Concept:
In commercial arithmetic, transactions involve Cost Price (C.P.), Selling Price (S.P.), and Profit/Gain percentage. A gain means the item was sold for more than its purchase price.
• Selling Price Formula for Gain:
\[ \text{S.P.} = \text{C.P.} \times \left(1 + \frac{\text{Gain \%}}{100}\right) \]
• Alternatively, \(\text{S.P.} = \text{C.P.} + \text{Profit Value}\), where \(\text{Profit Value} = \frac{\text{Gain \%}}{100} \times \text{C.P.}\)
Step 1: Identifying values given in the problem statement.
The parameters presented are:
Cost Price (C.P.) &= Rs. 1200
Gain Percentage &= 15%
Step 2: Calculating the actual absolute monetary gain value.
The financial profit made is \(15\%\) of the original purchase cost price:
\[
\text{Profit} = \frac{15}{100} \times 1200
\]
Simplifying the expression by cancelling out the double zeros in the numerator and denominator:
\[
\text{Profit} = 15 \times 12
\]
Let us carry out this multiplication systematically:
\[
15 \times 12 = 15 \times (10 + 2) = (15 \times 10) + (15 \times 2) = 150 + 30 = 180
\]
So, the total profit earned on the item is Rs. 180.
Step 3: Finding the final Selling Price.
We add the computed profit value directly to our core base cost price:
\[
\text{S.P.} = \text{C.P.} + \text{Profit}
\]
\[
\text{S.P.} = 1200 + 180 = 1380
\]
Thus, the retail selling price of the item must be set at Rs. 1380. This matches Option (D).