Past exam of the mathematics course of the University of Cambridge / 2014 / iii / Paper 37 / 4 / c / Solution

Introduce nonnegative surplus variablesAt the proposed starting basic feasible solution, the basic variables are $x_1,r_1,r_2$ and the nonbasic variables are $x_2,x_3,r_3$. Its simplex dictionary, with objective $z=x_1+x_2+x_3$, isIncrease $x_3$ to decrease $z$. The simplex ratio test gives limits $1/5$ from $x_1$, $1/21$ from $r_1$, and $3/31$ from $r_2$. The smallest is $1/21$, so $x_3$ enters and $r_1$ leaves. After this single pivot the dictionary isAll objective coefficients of nonbasic variables are strictly positive. Thus the simplex method has reached its unique optimum:The feasible dual vector $y=(1/21,0,4/21)$ has the same objective, providing an independent weak duality certificate. Normalization givesThese are all the equilibria: the dual's strict slack in row two forces $p_2=0$, and the primal's strict slack in column two forces $q_2=0$. Equality of the two active row and column payoffs then fixes the displayed probabilities. The value is the first player's expected net loss; the second gains $4/5$.