Table:
text: United States Senior Open
Winners
Winners
Year_ Champion Country Venue Location_ Score Winner_s_share_
1985 Miller Barber (3) United States Edgewood Tahoe Golf Course Stateline, Nevada 285 (−3) 40199
1984 Miller Barber United States Oak Hill Country Club , East Course Pittsford, New York 286 (−2) 36448
1983 Billy Casper United States Hazeltine National Golf Club Chaska, Minnesota 288 (+4) 30566
1982 Miller Barber United States Portland Golf Club Portland, Oregon 282 (−2) 28648
1981 Arnold Palmer United States Oakland Hills Country Club , South Course Birmingham, Michigan 289 (+9) 26000
1980 Roberto De Vicenzo Argentina Winged Foot Golf Club , East Course Mamaroneck, New York 285 (+1) 20000

Question: What is the average win percentage won by Billy Casper at the United States Senior Open?