Step 1: Observe the given figure for the hints

              NOTE:  triangle is a equilateral triangle

                           Inscribed circle with in-center O

                          AOP triangle is a right triangle

Step 2: Find the area of the right triangle

                             \frac{1}{2} (b * h)

Step 3:  Find the base and height of the right triangle

                  NOTE: OP is perpendicular to AC at middle point

                          Base b = \frac{side}{2}   = \frac{10}{2}

                          Height h = radius of the inscribed circle = side *\frac{\sqrt{3}}{6}

                         h = 10 * \frac{\sqrt{3}}{6}

Skill 3: Plugging the base and height values in the formula

                             \frac{1}{2} *\frac{10}{2}*10 * \frac{\sqrt{3}}{6}

                             25 \frac{\sqrt{3}}{6}