At noon, ship A is 150 km west of ship B. Ship A is sailing east at 35 km/h and ship B is sailing north at 30 km/h. How fast is the distance between the ships changing at 4:00 PM?

Answer 1

the distance z between the ships after t hours, is given by

z^2 = (150-35t)^2 + (30t)^2
at t=4, z=30√5

So, plug in your values, using

2z dz/dt = -70(150-35t) + 1800t

Answer 2

where does the -70 come from?