You have $20 to spend on taxi fare. the ride costs $5 plus $2.50, per mile.

I am assuming you are asking how many miles you can travel on the taxi.

In this situation a good method would be to make an equation.

Let x represent the number of miles traveled.
The ride costs $2.50 per mile, so the cost of the total trip is equal to 2.5x + 5. (the 5 is the base 5$ cost)

Set this equation equal to 20 as you have $20 to spend.

So, we get
2.5x + 5 = 20
Subtract 5 on both sides to get:
2.5x = 15
Divide by 2.5 on both sides to find x. 
x = 6

You can travel 6 miles in the taxi.