If a vendor has correctly used marginal analysis to select their stock levels for the day $($as in the newsperson problem$),$ and the profit resulting from the last unit being sold $(C_u)$ is $$0\mathrm{.}90$ and the loss resulting from that unit if is not sold $(C_0)$ is $$0\mathrm{.}50,$ which of the following is the probability of the last unit being sold?