Volume measures the space inside a solid, while surface area adds up the areas of every face, and Surface area and volume of solids tests both.
Expect rectangular boxes, cylinders, cones and spheres; the ACT usually supplies the less familiar formulas, such as the volume of a sphere, four-thirds pi r cubed. Some questions run a formula backward, giving the volume and asking for the radius or diameter.
Scaling is a favorite twist: multiplying every length by k multiplies area by k squared and volume by k cubed, not by k. Painting or wrapping problems may leave out a hidden face, so read which surfaces count.
Sample questions
Pick an answer to see whether it is right — nothing is saved, and nothing needs an account.
Question 1Easier
A rectangular wooden block measures 12 inches by 7 inches by 5 inches. Every face except the 12-inch-by-7-inch bottom is painted. What is the area, in square inches, of the painted surface?
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Answer: C — 274
All 6 faces have area 2(12(7)+12(5)+7(5))=358 square inches. The unpainted bottom has area 12(7)=84, so the painted area is 358−84=274 square inches.
Question 2Mid
A rectangular container measures 5 by 3 by 2 feet. A similar container has every linear dimension multiplied by 1.6. To the nearest cubic foot, what is the larger container’s volume?
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Answer: C — 123
Volumes of similar solids scale by the cube of the linear scale factor. The new volume is 30(1.6)3=122.88, which is approximately 123 cubic units.
Question 3Mid
A sphere has a volume of 288π cubic centimeters. What is the diameter of the sphere, in centimeters?
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Answer: C — 12
Solving 34πr3=288π gives r3=216, so r=6 centimeters. The diameter is 2r=12 centimeters.
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