Control Data Corp. will buy a 20%...

Control Data Corp. will buy a 20% stake in Silicon Graphics Inc. for $68.9 million in a step intended to cement their business relationship, the computer companies said. Minneapolis-based Control Data said it intended to buy $150-million worth of computer work stations from Silicon Graphics in the next three years, continuing an arrangement with the Mountain View, Calif.-based company that dates back to late 1986. The companies will cooperate on developing new work stations. Control Data will buy newly issued shares of common stock in Silicon Graphics for $22.50 a share that will represent 20% of the company’s stock.