12 mph faster than 60mph is 60+12=<<60+12=72>>72 mph.
18 mph slower than 60 mph is 60-18=<<60-18=42>>42 mph.
If he spends 1/3 of the time at each of these three speeds, then he averages (60+72+42)/3 = <<(60+72+42)/3=58>>58 mph.
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