arithmetics
Anyone that kows how to resolve this problem mentally?. It's easy to do it if you do it writen with a linear equation but doing that is not feasible since it takes too long...
A six passenger plan cruises at 180mph in calm air. If the plane flies 7 miles with the wind in the same amount of time as it flies 5 miles against the wind, then what is the wind speed?.
The solution is apparently 30mph...