The Agriculture Department reported that U.S. rice exports are expected to be up around 7% in the 1988-89 marketing year, rebounding from a 14% decline last season . . . Times Mirror Co., Los Angeles, said it has reached an agreement to purchase the stock of Chillicothe Cablevision Inc., which has 29,000 cable-TV subscribers in Ohio, for an undisclosed price . . . Baker Hughes Inc. of Houston reported that the number of working oil and gas rigs this week jumped by 11 to total 925. A year ago, the count was 1,134 . . . The Federal Reserve said the average yield for one-year T-bills, the most popular index for making changes in adjustable-rate home mortgages, fell to 8.07% last week after averaging 8.13% the week before . . . American City Business JournalsInc., the Kansas City, Mo., publisher of 21 weekly business newspapers, said it will bebought by TA Associates Communications of Boston in a dealworth between $124 million and $160 million.