I’m not that good at math, it’s my worst subject actually...but I believe they drove 5 miles. My first thought was 6 miles, mostly because of the $2 but the added $1 would’ve been too high. If the two people drove 5 miles, that would be $10. Since two people are in the taxi, that would mean that they each payed the extra $1. That would make $2 to add to the $10 which would create the correct amount of $12.
Answer: 5 miles