Question 23363
Try this:

{{{d1 = r1t1}}} This is the distance traveled on hwy 401 where r1 is 100 km/h
{{{d2 = r2t2}}} This is the distance traveled on hwy 7 where r2 is 60 km/h

The total distance is d1 + d1 = 480 km.
The total time is t1 + t2 = 6 hours. Rewrite this as: t2 = 6-t1 

{{{d1 = 100(t1)}}}
{{{d2 = 60(6-t1)}}} Here, you have substituted for (6-t1) for t2

{{{d1 + d2 = 480}}} so:
{{{100(t1) + 60(6-t1) = 480}}} Simplify and solve for t1
{{{100t1 + 360 - 60t1 = 480}}} Collect like-terms.
{{{40t1 + 360 = 480}}} Subtract 360 from both sides.
{{{40t1 = 120}}} Divide both sides by 40.
{{{t1 = 3}}}hours.

Now you can find d1, the distance traveled on hwy 401.

{{{d1 = 100(t1)}}}
{{{d1 = 100(3)}}}
{{{d1 = 300}}}Km. The distance traveled on hwy 401.