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.