See the attached picture:
RO divides the rectangle into two congruent right triangles.
The area of the one triangle is equal half area of the rectangle.
Calculate the area of rectangle:

The area of right triangle:

Use the Pythagorean theorem to calculate the length of RO:

The formula of an area of this right triangle is:

Therefore we have the equation:

Answer
Find out the what fraction of the sandcastle did they build together .
To prove
Let us assume that the fraction of the sandcastle they build together be x .
As given


Than the equation becomes

L.C .M of (3,7) = 21
solving


Therefore the fraction of the sandcastle did they build together be

Hence proved
Supplementary angles are angles that add up to 180 degrees, aka one straight line. The supplementary angles here are:
1 and 2, 1 and 3, 2 and 4, 3 and 4, 5 and 6, 6 and 8, 7 and 8, and 6 and 7. Well, those are the more obvious ones. 1 and 7 are supplementary because if you envision them next to each other, you’ll see that they create a straight line. So, with that logic, 3 and 5 are supplementary because when you put them together, they create a straight line