The roller coaster will need to run 4 times.
Explanation:
If there are 15 adults and 9 children, then we know that they're 24 people in total on the line (15 + 9 = 24). Now, we divide 24 by 6, because there are 24 people and a roller coaster holds up to 6 people. 24/6 = 4, therefore the roller coaster will need to run 4 times.
The midpoint is (-1/2, 2)
I wonder what shape the window is? It has a radius so I expect a circle but you use the word perimeter which is usually reserved for shapes with sides.
So my guess is you are looking for the circumference of a circular window with radius 20. The circumference is given by (pi)d where d is the diameter. The diameter is twice the radius so 40. We use 3.14 to approximate pi.
That makes the circumference (the distance around the circle) = (40)(3.14)=125.6