At a national seashore, the number of swimmers per day who require lifeguard assistance averages 4.1. On any day when 7 or more swimmers are assisted, the lifeguards receive a $20 bonus for extra effort. Bob's base life-guarding pay is $12 per hour for each eight-hour day. Taking into account the extra-effort days that sometimes occur, what is Bob's average pay for an eight-hour day?