math

During the Sydney to Hobart Yacht Race the yacht Adelaide starts taking on water. Two other yachts are in the vicinity and are called for help: the yacht Eloise is exactly 40 kilometres south of the Adelaide and the yacht Matilda is exactly east of the Eloise and 80 kilometers away from the Adelaide. Both yachts start travelling directly towards the Adelaide, the Eloise with a constant speed of 40 kilometres per hour and the Matilda, due to a strong easterly wind, much faster with 60 kilometres per hour. What is the rate of change of the distance between the Eloise and the Matilda at the moment when the Eloise is 20 kilometres away from the Adelaide? Hint: Draw a sketch and use the cosine rule