Answer:
475 mph
Step-by-step explanation:
Distance = rate × time
time = distance / rate
If x is the speed of the plane in still air:
2100 / (x + 25) = 1890 / (x − 25)
Cross multiply:
2100 (x − 25) = 1890 (x + 25)
Distribute and solve:
2100x − 52500 = 1890x + 47250
210x = 99750
x = 475