The solution to this question requires analysis of mutually exclusive (conditional) sets. Each set has an individual probability. The sum of these probabilities times the probability of choosing one of the sets determines the overall probability of a “factorific” coloring.
From the question: The condition requires all divisors of a blue number to be blue. Note a number can be blue without it being a divisor of all greater numbers as long as its divisors are blue.
*Grogg chooses to color none (0) of the numbers.
Probability of factorific: Zero (0)
*Grogg chooses to color one (1) of the six (6) numbers.
Probability of colorific: (1/6).
Explanation: Number one (1) is the only positive integer that has one divisor.
*Grogg chooses to color two (2) of the six (6) numbers.
Probability of factorific: = (1/5)
Explanation: Number of ways to choose two numbers from 6 numbers = (nCr(6, 2)) = 15.
Number one (1) must be in the set because it’s a divisor of all integers. The other number has to be a prime number (2, 3, or 5)
{1, 2}
{1, 3}
{1, 5}
Number of sets that have a one (1) and a prime: Three (3).
Probability (3/15) = (1/5)
*Grogg chooses to color three (3) of the six (6) numbers.
Probability of factorific: (5/20) = (1/5)
Explanation: Number of ways to choose 3 numbers from 6 numbers = (nCr(6, 3)) = 20
Number one (1) must be in the set because it’s a divisor of all integers.
These 4 sets meet the conditions:
{1, 2, 3} Two (2) is blue here, it’s not a divisor of three, but one (1) is its divisor, so this is valid.
{1, 2, 4} all divisors are blue for all blue numbers.
{1, 2, 5} ‘’
{1, 3, 5} ‘’
{1, 5, 6} ‘’
Five of 20 sets meet the conditions: (1/5)
*Grogg chooses to color four (4) of the 6 numbers.
Probability of factorific: (4/15)
Explanation: Number of ways to choose 4 numbers from 6 numbers = (nCr(6, 4)) = 15
Number one (1) must be in the set because it’s a divisor of all integers.
These 3 sets meet the conditions
{1, 2, 3, 4} all divisors are blue for all blue numbers.
{1, 2, 3, 5} ‘’
{1, 2, 3, 6} ‘’
{1, 2, 4, 5} ‘’
Four of 15 sets meet the conditions: (4/15)
*Grogg chooses to color five (5) of the six (6) numbers.
Probability of factorific: (1/2)
Explanation: Number of ways to choose 5 numbers from 6 numbers = (nCr(6, 5)) = 6
Number one (1) must be in the set because it’s a divisor of all integers. Number of sets of five (5) that have one (1) as an element = (nCr(5, 4)) = 5.
Three sets meet the conditions
{1,2,3,4,5} all divisors are blue for all blue numbers.
{1,2,3,4,6} ‘’
{1,2,3,5,6} ‘’
Three sets of 6 sets meet the conditions (3/6) = (1/2)
*Grogg chooses to color six of the six (6) numbers.
Probability of factorific: (1) or 100%
Explanation:All numbers are blue and all numbers have all their divisors colored blue.
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Sum of individual (mutually exclusive) probabilities: ((0)+(1/6)+(1/5)+(1/4)+(4/15)+(1/2)+(1))
Grogg has a (1/7) probability of picking one of the seven sets (including the empty set).
(1/7)*((0)+(1/6)+(1/5)+(1/4)+(4/15)+(1/2)+(1)) = (143/420) ≈ 34.05%
(1/7)*((0)+(1/6)+(1/5)+(1/5)+(4/15)+(1/2)+(1)) = (143/420) ≈ 33.33%
The overall probability that Grogg’s coloring is factorific is ≈ 33.33%
Sources: A genetically enhanced chimp brain and comprehensive programming of basic set theory from Lancelot Link.
GA
Edits: Error corrections
Another solution and more comments here.
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Heureka, your answer is correct if a box is restricted to a maximum of one piece of candy.
nCr(4, 1) + nCr(3, 2) = 12
Reasoning:
There are nCr(4, 1) = 4 ways to place the unique piece of candy in one of the four (4) boxes. Then the 2 indistinguishable candies can be placed in the other 3 boxes nCr(3, 2) = 8 ways. There are (4+8) = 12 ways to distribute the candies.
If there are no restrictions on the maximum then:
\(\hspace{15pt} \left( \dfrac{4!}{2!}\right)* \left(2 \right) \;+\; \left( \dfrac{4!}{2!}\right) \;+\; \left(4\right)\\ \text {Reasoning:}\\ \text {All candy in 1 of 4 boxes (4 ways)}\\ \text {2 combinations (XX|Y & XY|X) distributed to boxes in (2)(4*3) = (2)(4!/2!) =(24) ways;}\\ \text {and 1 candy to each box distributed to boxes in (1)(4*3*2)/2 = 4!/2! = (12) ways.} \\ \text {Total 4+24+12 = 40 ways.} \)
(Adapted from Nauseated’s solution https://web2.0calc.com/questions/i-really-need-help-please-i-have-no-idea-how-to-do-it-sorry#r4)
GA