math
posted by lisa .
At noon, ship A is 30 nautical miles due west of ship B. Ship A is sailing west at 21 knots and ship B is sailing north at 15 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.)
this is a cal problem.

after 4 hours
x =  30  21*4 =  114
y = 15*4 = 60
dx/dt = 21
dy/dt = 15
D at 4 hr = sqrt(114^2+60^2) = 129
D^2 = x^2 + y^2
2 D dD/dt = 2 x dx/dt + 2 y dy/dt
129 dD/dt = 114(21) + 60(15)
dD/dt = 25.5 knots 
i think ur answer is wrong

I got the same answer as Damon.
Damon is right. 
i type it in the computer , its says its wrong. but thanks