At midnight, ship B was 90 km due south of ship A. Ship A sailed east at 15 km/hr and ship B sailed north at 20 km/hr. At what time were they closest together?

Answer 1

at time t, the distance d is

d^2 = (90-20t)^2 + (15t)^2
d^2 = 625t^2 - 3600t + 8100

2d dd/dt = 1250t-3600
dd/dt=0 when t=72/25
d(72/25) = 54