Logically you can see that since 26 is half way between 24&28 it is 1/2 sd.
See sketch
isosceles trapezoid
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Volume of a cone = (PI * radius^2 * height) / 3
Volume of a cone = (PI * 3^2 * 8) / 3
Volume of a cone = (PI * 24)
Cylinder Volume = PI • r² • height
Cylinder Volume = PI * 3^3 * 12
Cylinder Volume = PI * 108
TOTAL Volume = PI * 132