On highways, the Jensen family used 210 miles / 35 miles/gallon = <<210/35=6>>6 gallons of gas.
On city streets, the Jensen family used 54 miles / 18 miles/gallon = <<54/18=3>>3 gallons of gas.
Total, the Jensen family used 6 gallons + 3 gallons = <<6+3=9>>9 gallons of gas.
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