Hallo,

t is the time they traveled at 25 mph.

Than is (5-t) the remaining time.

During this remaining time they travel at a speed of 50 mph.

After 200 miles the journey is completed:

That means: They travel 2 hours at 25 mph: 50 miles

and 3 hours at 50 mph: 150 miles.

Adds up to a total of 200 miles.