Answer:
dx/dt = 4 mph
Step-by-step explanation:
The two trains, with the station ( origin of the coordinates system) all, form a right triangle when we considered the distance between the two trains.
In that triangle:
distance between trains D = 24 miles
leg (1 ) distance of train going west to origin y = 15 miles
leg (2) distance of train going south x = ??
When d = 24 and y = 15
D² = y² + x² ⇒ x² = D² - y²
x = √ (24)² - (15)²
x = √351
x = 18,73 miles
Now according to Pythagoras theorem
D² = x² + y²
Taking derivatives (respect to time) on both sides of the equation we get
2*D*dD/dt = 2*y*dy/dt + 2*x*dx/dt
In this expression we are looking for dx/dt, all the rest of variables are known
2*24*25 = 2* 15*35 + 2*(18,73)* dx/dt
(1200 - 1050 )/37,46 = dx/dt
dx/dt = 4 mph