If a vendor has correctly used marginal analysis to select its stock levels for the day $($as in the newsperson problem in the text$),$ and if the profit resulting from the last unit being sold $($Cu$)$ is $$0\mathrm{.}90$ and the loss resulting from that unit if it is not sold $($Co$)$ is $$0\mathrm{.}50,$ which of the following is the probability of the last unit being sold?