calculate the time it took him to drive:
he drove 150 miles at 50 mph
150/50 = 3 hrs
150 miles at 30 mph = 150/30=5 hrs
so he drove a total of 5+3 = 8 hours
he drove a total of 150 +150 =300 miles
300/8 = 37.5 miles per hour average for entire trip
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