We dont know the average speed. Hence the time, spent in first 200 km does not make any sense.
vt = 400 -------(1)
vt = 200 = (v+ 20) (t - 1)
vt = vt + 20t - v - 20
20t = v + 20
Multiply both sides by t
20t^2 = vt + 20t
20t^2 = 400 + 20t -----(we know vt = 400)
t^2 = 20 + t
hence t^2 - t - 20 = 0
This will gives us two roots one positive and one negative. Hence t has unique value. Sufficient
How much time did it take a certain car to travel 400 kilometers?
1) The car traveled the first 200 kilometers in 2.5 hours.
2) If the car's average speed had been 20 kilometers per hour greater than it was, it would have traveled the 400 kilometers in 1 hour less time than it did.