EXHIBIT 12 COMPUTATION OF RATIO OF EARNINGS TO FIXED CHARGES Three Months Ended Mar. 31 Mar. 31 1998 1997 ------- ------- (Thousands of dollars) Income before income taxes, extraordinary item and cumulative effect of accounting changes. $79,508 $66,992 Amortization of capitalized interest 610 530 Interest expense 5,863 5,465 Interest portion of rental expense 607 655 ------- ------- Earnings $86,588 $73,642 ======= ======= Interest $ 7,076 $ 5,692 Interest portion of rental expense 607 655 ------- ------- Fixed Charges $ 7,683 $ 6,347 ======= ======= Ratio of Earnings to Fixed Charges 11.27 11.60 ======= =======