Independent solution

How to solve this Combinatorial Probability question

Setup

Setup

Partition the nine members into six target members and three others. Every four-member subset is equally likely.

#{all samples}=(94)=126\#\{\text{all samples}\}=\binom94=126

Model

Model

At least three target members means either exactly three or exactly four, and these two events are disjoint.

Pr(X3)=(63)(31)+(64)(30)(94)\Pr(X\ge3)=\frac{\binom63\binom31+\binom64\binom30}{\binom94}

Compute

Compute

Evaluate the two favorable counts and reduce the result.

(63)(31)=60,(64)(30)=15\binom63\binom31=60,\qquad \binom64\binom30=15
Pr(X3)=60+15126=2542=0.5952380952\Pr(X\ge3)=\frac{60+15}{126}=\frac{25}{42}=0.5952380952\ldots

Answer

Answer

The probability rounds to 0.60.

0.60(E)\boxed{0.60\quad\text{(E)}}