Luisa drives a total of 10 + 6 + 5 + 9 = <<10+6+5+9=30>>30 miles
Luisa uses 30 / 15 = <<30/15=2>>2 gallons of gas for her trip
The total cost of the gas for Luisa’s trip is 2 * $3.50 = $<<2*3.5=7>>7
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