Question 138689
What is the same for both vehicles? That's aways the key question 
in these problems. It is elapsed time until they meet
For the truck:
{{{d = r*t}}}
{{{d = 45t}}}
{{{t = d/45}}}
For the car:
{{{800 - d = 60t}}}
Now use substitution
{{{800 - d = 60(d/45)}}}
{{{45*800 - 45d = 60d}}}
{{{36000 = 105d}}}
{{{d = 342.86}}}, or to the nearest mile. 343 mi
{{{800 - 343 = 457}}}
The truck travels 343 mi and the car travels 457 mi before they meet