Chapter 1, Equations and Inequalities - 1.5 Use Problem Solving Strategies and Models - Guided Practice for Examples 2,3, and 4 - Page 36: 4

$5$ gallons were used in the city.

Let $x$ be the number of gallons used on the highway. Then the number of gallons used in the city is $28-x$. Then our equation is: $16x+(28-x)12=428\\16x+336-12x=428\\4x=92\\x=23$. Thus $28-23=5$ gallons were used in the city.