To evaluate how much space Robert will have left, we first need to calculate the area of one square with a side length of 7 inches. The area of a square is given by the formula:
\[ \text{Area} = s^2 \]
where \( s \) is the length of the side. So:
\[ s^2 = 7^2 = 49 \text{ square inches} \]
Since Robert is using 10 squares, the total area occupied by the squares is:
\[ 10s^2 = 10 \times 49 = 490 \text{ square inches} \]
Now, we can find out how much space Robert will have left by subtracting the total area of the squares from the area of the poster board:
\[ 720 - 10s^2 = 720 - 490 = 230 \text{ square inches} \]
Thus, the correct answer is:
230 square inches.