Two cars are at an intersection. One goes east at a speed of 40 feet per second; the other goes north at a rate of 60 feet per second. When the first car is 240 feet from the intersection (and hence...

Two cars are at an intersection. One goes east at a speed of 40 feet per second; the other goes north at a rate of 60 feet per second. When the first car is 240 feet from the intersection (and hence the other is 360 feet away), at what rate is the distance between them changing? You may use a calculator.