The average would be the total amount divided by the 2 groups, so the answer would be 37.5 years old.
You can see if the other angles add up to 180 or 360 degrees (depending on shape) then add them to make it the number, for example, if a triangle has a right angle, then the other 2 angles are 45 degrees, knowing that EVERY triangle's angles add up to 180 degrees.
As the word suggests, a quarter-sphere is a quarter of a sphere, and thus the volume of a quarter-sphere is a quarter of the volume of the sphere.
The volume of a sphere is computed as

So, the volume of a quarter-sphere is
