To find the total area of the octagon you can find the area of the rectangle that is created by the entire figure and subtract the areas of the four congruent right triangles.
(10 cm x 6 cm) - [4(1/2 x 2 x 2)] 60 cm² -8 cm² = 52 cm²
If you multiply 15 by 2 you were to get 30 but since it is 15 multiplied by 20 instead its like adding a 0 at the end of the number so we change 30 to 300.
