a When working with speed, it's often helpful to use a table and the equation, d=rt, as a basis for our equation. In this case, we are solving for time, so t= dr. Let's start by organizing our information in a table.
Distance, d
Rate, r
Time, t=d/r
Interstate
150
x
150/x
Highway
130
x-10
130/x-10
Total Trip
280
70
4
From the table, we can see the expression for time on the interstate is 150x and on the highway is 130x-10.
b The equation to find the speed limit uses the information that the total trip took 4 hours. We can use the expressions for time that we developed in Part A to form our equation.
