The original cube has faces with area 100 square inches, so those faces must have sides of length 10. That would give the cube a total volume of 10 ^ 3 = 1,000 cubic inches.
Since the new cube is one third the volume of the original, it has volume 1,000 / 3 = 333.33 cubic inches.
Now we have to figure out what side length would give a cube a volume of 333.33 cubic inches. Lets try some numbers from the answer choices..
= 7 * 7 * 7
= 49 * 7 =343.
Too big, but close.
= 6 * 6 * 6
= 36 * 6
Way too small.
All the other choices are even smaller, so the closest answer is 7 and the answer is A
NOTE: The thing to understand about problems like this is that in proportional three dimensional shapes, if the volume changes by some amount, the linear measures will always change by the cubic root of that amount. In this case, the volume of the sphere changed to be 1/3 of its original, so the linear change is the cubic root of 1/3. Multiplying 10 by the cubic root of 1/3 gives 6.9 which is almost 7. Our answer checks.
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