Hello, Chris!
A good sketch helps, of course . . .
Two cars travel on perpendicular roads towards the intersection of the roads.
The first car is 100 miles from the intersection and is traveling at 55 mph.
The second car starts at the same time 250 miles from the intersection and is going 60 mph.
How fast is the distance between them changing 1.5 hours later? Code:
P o

55t 

A o
 *
 * x
10055t *
 *
 *
C o         o      o Q
25060t B 60t
The intersection is at
The first car starts at , where ,
. . and drives south at 55 mph.
In hours, it moves miles to
. .
The second car starts at , where ,
. . and drives west at 60 mph.
Inb hours, it moves miles to
. .
The distance between them, ,
. . is the hypotenuse of right triangle
Therefore: .
Got it?