<PRE><B>Question 664849</B>
Plane A 12 mph
Plane B = 9 mph

They fly at right angles to each other.

after 1/3 hour plane will be 1/3 * 9 = 3 miles from origin
after 1/3 hours plane B will be 1/3 * 12 = 4 miles from origin.

They form a right triangle with origin at at any moment.

D^2 = 12^+9^2

d^2= 144+81
d^2= 225 
d= sqrt(225)
d= 15 miles

They will be 15 miles apart.</PRE>