Leila and her friends will travel a total distance of 150 x 2 = <<150*2=300>>300 kilometers back-and-forth.
They will need 300/15 = <<300/15=20>>20 liters of gasoline for this trip.
So, they will pay $0.90 x 20 = $<<0.90*20=18>>18 for the gasoline.
Thus, the first option will costs them $50 + $18 = $<<50+18=68>>68.
Therefore, they can save $90 - $68 = $<<90-68=22>>22 if they choose the first option.
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