Question

Al, Bill, and Cal will each randomly be assigned a whole number from $1$ to $10,$ inclusive, with no two of them getting the same number. What is the probability that Al's number will be a whole number multiple of Bill's and Bill's number will be a whole number multiple of Cal's?

$\dfrac{9}{1000}$

$\dfrac{1}{90}$

$\dfrac{1}{80}$

$\dfrac{1}{72}$

$\dfrac{2}{121}$

Answer: C

Concepts:basic probability divisibility systematic listing

Difficulty rating: 1600

Solution:

Let $(A,B,C)$ be the numbers assigned to Al, Bill, and Cal. We need $A$ to be a multiple of $B$ , and $B$ to be a multiple of $C$ , with all three numbers distinct.

The valid triples are $\begin{gathered} (4,2,1),(6,2,1),(8,2,1), \\ (10,2,1),(6,3,1),(9,3,1), \\ (8,4,1),(10,5,1),(8,4,2). \end{gathered}$ There are $9$ favorable assignments.

The total number of assignments is $10\cdot9\cdot8=720$ , so the probability is $\frac9{720}=\frac1{80}$ .

Thus, the correct answer is C.

2015 AMC 10B Problem 16

Answer: C

Difficulty rating: 1600

Solution:

Problem 16 in Other Years