Berkshire acquired its 80% stake in Pilot Travel Centers, or PTC, over a six-year period for $11 billion. Starting this month, Pilot Corp. was to have an annual 60-day window in which it could require Berkshire to purchase its remaining 20% stake. The price, in the event Pilot Corp. exercised its option, had been set at a contractually agreed formula of 10 times the prior year’s earnings before interest and taxes, with some adjustments.