Explanation:

- The average cost price of the two shirts is Rs. 400, so the total cost for both is Rs. 800.

- Shirt A has a profit of 10%. If its cost is \( x \), the selling price becomes \( x + 0.1x = 1.1x \).

- Shirt B has a profit of 20%. If its cost is \( y \), the selling price becomes \( y + 0.2y = 1.2y \).

- The total selling price of both shirts is Rs. 928, thus:

\( 1.1x + 1.2y = 928 \).

- From the total cost equation \( x + y = 800 \), solve for \( y \).

- Rearranging gives \( x = 800 - y \).

- Substituting into the selling price equation:

\( 1.1(800 - y) + 1.2y = 928 \),

simplifying to \( 880 - 1.1y + 1.2y = 928 \),

further simplify to \( 0.1y = 48 \) leading to \( y = 480 \).

- Hence, the cost price of shirt B is Rs. 480.

- Option 3: Rs. 480 is the correct option.