First find 1/3 of the distance Jenny's bottlecap went straight: 18 feet / 3 = <<18/3=6>>6 feet
Then add that to the distance it flew straight to find the total distance it traveled: 6 feet + 18 feet = <<6+18=24>>24 feet
Then find twice the distance Mark's bottlecap went straight: 15 feet * 2 = <<15*2=30>>30 feet
Then add that distance to the distance it flew straight to find the total distance it traveled: 30 feet + 15 feet = <<30+15=45>>45 feet
Then subtract the distance Jenny's bottlecap traveled from the distance Mark's traveled to find the difference: 45 feet - 24 feet = <<45-24=21>>21 feet
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