Question 504210
d = r*t is the basic distance equation
.
Jack's rate = 70 mph
Robert's rate  = 30 mph
.
When Jack and Robert cross paths, they will have been traveling for the same amount of time. They will have traveled different distances, of course.  But they will have been on the road and equal amount of time. In addition, when they cross paths, their combined distance traveled will = 200 miles.
.
30t + 70t = 200 miles
100t = 200
t = 2 hr
.
Always check the solution.
.
Jack will have traveled 2 hr *70 mph = 140 miles.
Robert will have traveled 2 hr * 30 mph = 60 miles.
OK.
.
Answer: They will cross paths in 2 hr.
.
Done.