[tex]\\\text{Side length of square A = a \text{ft}}\\\\\text{Side length of square B = b \text{ft}}\\\\ \text{Side length of square C = c \text{ft}}\\\\ \text{Given that,}\\\\\text{Area of square C} = 400 ~ \text{ft}^2\\\\\implies c^2=400~~~~~~......(i)\\\\\\\text{Perimeter of square B } = 64~ \text{ft}\\\\\implies 4b = 64\\\\\implies b = \dfrac{64}4\\\\\implies b = 16\\\\\implies b^2 = 256~~~~~~~.....(ii)\\\\\\[/tex]
[tex]\text{Applying Pythagorean theorem,}\\ \\~~~~~c^2 =a^2 +b^2\\\\\implies a = \sqrt{c^2 - b^2}\\ \\\implies a = \sqrt{400-256}\\\\\implies a = \sqrt{144}\\\\\implies a = 12[/tex]
[tex]\text{Hence, the side length of square A is 12 ft.}[/tex]
