To take a taxi , it cost $3.00 plus an additional $2.00 mile traveled. you spent exactly $20 on a taxi, which includes the $1tip you left. how many miles did you travel?

Let's break it down. 
You spent $20. Plus the $1 tip. 
20 - 1 = 19 
$19 is how much you spent without the tip
The cost for the taxi ride itself is $3
19 - 3 = 16
$16 is how much you spent without the ride cost + tip
You spent $2 for every mile traveled.
16/2=8 

You traveled 8 miles