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When to stop drawing from 2 white and 3 black

Question

An urn holds 22 white and 33 black balls. Jesse draws balls one by one without replacement and may stop whenever he likes (including before the first draw). Each white ball drawn earns him $1\text{\char36}1 and each black ball drawn costs him $1\text{\char36}1. Under the best stopping rule, what is his expected net result?

Short answer

A gain of 15\tfrac15 dollars. Let V(w,b)V(w,b) be the value with ww white and bb black balls left: V=max⁡{0, ww+b(1+V(w−1,b))+bw+b(V(w,b−1)−1)}V=\max\{0,\ \tfrac{w}{w+b}(1+V(w-1,b))+\tfrac{b}{w+b}(V(w,b-1)-1)\}. Working up from small urns (V(1,1)=12V(1,1)=\tfrac12, V(1,2)=0V(1,2)=0, V(2,2)=23V(2,2)=\tfrac23, V(2,1)=43V(2,1)=\tfrac43) gives V(2,3)=15V(2,3)=\tfrac15, so drawing is worthwhile.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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