Answer:
160 mph
Step-by-step explanation:
The relationship between time, speed, and distance is ...
time = distance/speed
__
If p represents the plane's speed, then the speed with the wind is p+20 and the time to go 405 miles is ...
405/(p+20)
The speed against the wind is p-20, and the time to go 315 miles is ...
315/(p-20)
These two times are equal, so we have ...
405/(p+20) = 315/(p-20)
405(p-20) = 315(p+20) . . . . . . cross multiply
90p = 6300 +8100 = 14400 . . . . add 8100 -315p
p = 14400/90 = 160
The speed of the plane in still air is 160 mph.
