Let's use basic logic here. If we are 100 miles away from a place, and we've got 25 miles to go, we've already travelled 75 miles. Since we've travelled 75 miles in 3 hours, then we travel at a rate of 75 divided by 3 = 25 miles per hour.
The distance to City A will be represented as 100 - miles already travelled. We can find how many miles have been travelled by multiplying 25 by t, the amount of hours we've spent driving.
This creates the equation d = 100 - 25t Hope this helped!
