Answer: 41 inches by 6 inches
Explanation:
A poster is in form of a rectangle and the area is length × breadth.
If the area of the poster is 240in² with 1 inch margins at the bottom and sides and 2in margin at the top.
The extra length of the poster will be 1-in at the bottom plus 2-in margin at the top making additional height of 3-inches.
Also, the extra breadth the 1-in at the sides to give breadth of extra 2-inches in total.
The area of the extra length and breath will be 3inches × 2inches to give 6inches.
The area that will give the exact dimension will be 240inches+6inches = 246inches
The dimension an be 41inches by 6inches