At noon, ship A is 110 km west of ship B. Ship A is sailing east at 25 km/h and ship B is sailing north at 20 km/h. How fast is the distance between the ships changing at 4:00 PM?

Answer 1

Draw a figure!

At 4PM A has sailed 100 km east and is 10 km west of B's course.

B has sailed 80 km north.

The distance between them is sqrt(6500)

The equation you need is z (dz/dt)=x(dx/dt)+y(dy/dt) where z is the distance between them; x is the east west distance and y is the north direction.

Plug in the numbers and solve for dz/dt.