You have $20 to spend on a taxi fare. The ride costs $5 plus $2.50 per kilometer.

You have $20 to spend on a taxi fare. The ride costs $5 plus $2.50 per kilometer.

Let k be the number of kilometers.
Total Cost = Cost per kilometer * number of kilometers + Fixed Cost

With k for kilometers, 2.5 as cost per kilometer, and 5 as fixed cost, and 20 on total cost, we have:
2.5k + 5 = 20

To solve this equation for k, we type it in our math engine and we get
k = 6