math
posted by joy on .
When an airplane flies with a given wind, it can travel 3000 km in 5hrs. When the same airplane flies in the opposite direction against the wind it takes 10hrs to fly the same distance. Find the speed of the plane in still air and the speed of the wind.

speed of plane in still air  x km/h
speed of wind  y km/h
speed of plane with the wind  x+y km/h
speed of plane against the wind  xy km/h
first fact:
5(x+y ) = 3000
x+y = 600 #1 equation
second fact:
10(xy) = 3000
xy = 300 #2 equation
add #1 + #2, it becomes so easy. 
speed of a plane in still air is 450
speed of wind is 150