Gannett Co. disclosed that it is paying CBS Inc. $42.5 million for its previously announced acquisition of Family Weekly magazine, a 12.8-million circulation Sunday newspaper supplement . . . Du Pont Co. said it plans to increase its 1985 capital expenditures to $3.7 billion from $2.9 billion in 1984 . . . The common stock of Los Angeles-based Quotron Systems began trading on the London Stock Exchange.