11. An airplane flying against the wind travels 140 miles in the same amount of time it would take the same plane to travel 160 miles with the wind. If the wind speed is a constant 20 miles per hour, how fast would the plane travel in still air?

Plane speed x mph

wind speed 20 mph

against wind x - 20 mph

with wind x + 20 mph

Distance against wind 140

Distance with wind 160

Time = same equate the time against with time with current

t=d/r

140 /(x- 20 )= 160 /(x+ 20 )

140 *(x+ 20 )= 160 *(x- 20 )

140 x + 2800 = 160 x -3200

140 x -160 x = -3200 + -2800

-20 x = -6000

/ -20

x= 300 mph

just sum

10 + 10 = 20