A small plane travels 240 miles with the wind in the same ti

A small plane travels 240 miles with the wind in the same time it would take to fly 190 miles against the
wind. If the wind has a constant rate of 15 miles per hour, what is the speed of the plane in still air?

Solution

d=rt
240=(r+15)t or t=240/(r+15)
190=(r-15)t or t=190/(r-15)
Because the times are the same we set them equal:
240/(r+15)=190/(r-15) cross multiply.
240(r-15)=190(r+15)
240r-3,600=190r+2,850
240r-190r=2,850+3,600
50r=6,450
r=6,450/50
r=129 mph is the speed of the plane in still air.
Proof:
240/(129-15)=190/(129+15)
240/144=190/114
144*190=240*114
27,360=27,360