Cards numbered 1 to 200: A card is drawn uniformly at random. What is the probability the number on it is a perfect cube?

Difficulty: Easy

Correct Answer: 1/40

Explanation:


Introduction / Context:
We need the density of perfect cubes among the integers 1 through 200. Recognize that cubes grow quickly, so only a few lie within this range.


Given Data / Assumptions:
Numbers are 1,2,3,…,200 with equal probability; perfect cubes are n^3 for integer n ≥ 1.


Concept / Approach:
Find the largest n with n^3 ≤ 200. Then probability = (count of cubes)/200.


Step-by-Step Solution:

1^3 = 1, 2^3 = 8, 3^3 = 27, 4^3 = 64, 5^3 = 125, 6^3 = 216 > 200Cubes ≤ 200 are 1, 8, 27, 64, 125 ⇒ 5 numbersProbability = 5/200 = 1/40


Verification / Alternative check:
Use floor of cube root: ⌊∛200⌋ = 5 ⇒ count = 5; divide by 200.


Why Other Options Are Wrong:
3/40 and 7/40 miscount; 19/40 is not plausible given rarity of cubes.


Common Pitfalls:
Including 216 (6^3) erroneously or forgetting 1 is a cube.


Final Answer:
1/40

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