check this question and response from October.
Just change the numbers to fit yours.
http://www.jiskha.com/display.cgi?id=1255692321
At noon, ship A is 40 nautical miles due west of ship B. Ship A is sailing west at 18 knots and ship B is sailing north at 23 knots. How fast (in knots) is the distance between the ships changing at 4 PM? (Note: 1 knot is a speed of 1 nautical mile per hour.)
1 answer
1 answer