It's the standard equation, Distance = rate * time, but in one case you give the car a 1.5 hour head start. So let's just set up the equations and set them to the same distance when the pass each other.

We know the general equation Distance = Rate * Time, so for each vehicle we'll add a subscript to keep them straight; The distance is the same for both so it doesn't get a subscript.

Distance = Rate

Distance = Rate

We also know that the Time

Distance = Rate

Since the distance is the same, we'll set them both equations equal to each other;

Rate

Now let's plug in the numbers and solve

60 * Time

60 * Time

(60 - 40) * Time

20 * Time

Time

Now, If we carried the units through, it would look like this;

We still plug in the numbers and solve, but we keep the units along (just don't confuse them with variables)

60 MPH * Time

60 MPH * Time

(60 - 40) MPH * Time

20 MPH * Time

Time

. . 20 MPH

Time

See how the MPH cancel and leave the answer in hours? That gives us some assurance the equation was set up right. Now we can also check the answer by putting it back into the original equations;

Distance = Rate

Distance = Rate

So they check. Now we know it's correct.

We know the general equation Distance = Rate * Time, so for each vehicle we'll add a subscript to keep them straight; The distance is the same for both so it doesn't get a subscript.

Distance = Rate

_{bus}* Time_{bus}Distance = Rate

_{car}* Time_{car}We also know that the Time

_{car}= Time_{bus}+ 1.5 hours, since it had a head start, and we can substitute that into the other equation, just don't forget the parenthesis;Distance = Rate

_{car}* (Time_{bus}+ 1.5)Since the distance is the same, we'll set them both equations equal to each other;

Rate

_{bus}* Time_{bus}= Rate_{car}* (Time_{bus}+ 1.5)Now let's plug in the numbers and solve

60 * Time

_{bus}= 40 * Time_{bus}+ 40 * 1.560 * Time

_{bus}- 40 * Time_{bus}= 40 * 1.5(60 - 40) * Time

_{bus}= 40 * 1.520 * Time

_{bus}= 60Time

_{bus}= 60 / 20 = 3 HrsNow, If we carried the units through, it would look like this;

We still plug in the numbers and solve, but we keep the units along (just don't confuse them with variables)

60 MPH * Time

_{bus}= 40 MPH * Time_{bus}+ 40 MPH * 1.5 Hrs60 MPH * Time

_{bus}- 40 MPH * Time_{bus}= 40 MPH * 1.5 Hrs(60 - 40) MPH * Time

_{bus}= 40 MPH * 1.5 Hrs20 MPH * Time

_{bus}= 60 MPH*HrsTime

_{bus}= 60 MPH*Hrs. . 20 MPH

Time

_{bus}= 3 HrsSee how the MPH cancel and leave the answer in hours? That gives us some assurance the equation was set up right. Now we can also check the answer by putting it back into the original equations;

Distance = Rate

_{bus}* Time_{bus}= 60 MPH * 3 Hrs = 180 milesDistance = Rate

_{car}* Time_{car}= 40 MPH * 4.5 Hrs = 180 milesSo they check. Now we know it's correct.