28. Popeye the Sailor, during his voyage, arrives at the market on Ancient Nara Island. Initially, the amount of money he has is a two-digit number $\overline{a b}$. Within half an hour, Popeye spends half of his money, and the remaining amount is a two-digit number $\overline{c a}$, where $c$ is exactly half of $b$. Therefore, $\overline{a b}=$ . $\qquad$
$98$