so, the plane is already 500 miles west of A, so, to be 1920 miles away from A, it simply needs to keep on going for 1920 - 500, or 1420 more miles
now, the plane is going at 400miles in one hour, how long will it take it to cover 1420 miles?
well [tex]\bf \begin{array}{ccllll}
miles&hours\\
\textendash\textendash\textendash\textendash\textendash\textendash&\textendash\textendash\textendash\textendash\textendash\textendash\\
400&1\\
1420&x
\end{array}\implies \cfrac{400}{1420}=\cfrac{1}{x}[/tex]
solve for "x"
