Question

Driving from Chester to Boxerville, you see a sign that says you have seven miles to go. If the total length of the trip is 45 miles, how many miles have you already traveled?
What is the unknown quantity in the example?

Answer 1

45 - 7 = 38
unknown quantity is the miles already traveled.

Answer 2

For the first part,
$45 - 7 = x$
$x = 38 mi$

The unknown quantity was the distance you had already traveled.

Do you understand this? Or should I explain more explicitly?