It's quite simple when you break it down
20/45 let's divide by 5
4/9
we can't divide anymore, no common denominator. The answer is 4/9
To solve, make an equation. The equation would be

x represents the miles driven
to solve, subtract 4 from each side of the equation.
$32-4 = 4-4 + 2x
The equation is now
$28 = 2x
Now you divide by 2 on each side.
$28 ÷ 2 = 2x ÷ 2
simplify
14 = x
The answer is 14 miles
4 is the GCF.
Explanation:
4x3=12
4x5=20
5 Isn’t a factor of 12, and 6 isn’t a factor of 20. Each number above doesn’t work, so the greatest common factor of the numbers 12 and 20 must be 4.