Independent solution

How to solve this Sampling Without Replacement question

Setup

Setup

Use the four available colors to identify the latest possible first match.

Tmax=5T_{\max}=5
{T=5}={first four colors are all different}\{T=5\}=\{\text{first four colors are all different}\}

Model

Model

After each new color, count the remaining socks whose colors have not yet appeared.

Pr(T=5)=1(67)(46)(25)\Pr(T=5)=1\left(\frac67\right)\left(\frac46\right)\left(\frac25\right)

Compute

Compute

Multiply the three without-replacement continuation probabilities.

Pr(T=5)=674625=835=0.2285714\Pr(T=5)=\frac67\frac46\frac25=\frac8{35}=0.2285714

Answer

Answer

The maximum draw count is required with probability about 0.2286.

0.2286(E)\boxed{0.2286\quad\text{(E)}}