The cost of the Lyft ride is $22 - $3 = $<<22-3=19>>19
The original cost of the taxi ride is $19 - $4 = $<<19-4=15>>15
Trevor tips the taxi driver $15 * 0.20 = $<<15*0.20=3>>3
The total cost of the ride downtown is $15 + $3 = $<<15+3=18>>18
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