Answer:
At 4 hrs ship A is 180 W and 80 S (kilometers)
" " B is 160 N
L^2 = 180^2 + 240^2 km using Pythagorean theorem
(Note at 4 hrs L = 300 km)
L^2 = 180^2 + (20 + 40)^2 * 4^2 = 180^2 + t^2 * 60^2 distance at 4 hrs
2 L dL / dt = 2 t * 3600
dL / dt = 4 * 3600 / 300 = 48 km/hr