Apuppy and a kitten are 180 meters apart when they see each other. the puppy can run at a speed of 25 m/sec, while the kitten can run at a speed of 20 m/sec.
how soon will the puppy catch the kitten if the puppy starts running after the kitten?

Make two equations representing the position of each animal, p and k, for the puppy and kitten respectively.

p=25t and k=180+20t  

The puppy will catch the kitten when p=k so we can say:

25t=180+20t  subtract 20t from both sides

5t=180  divide both sides by 5

t=36

So the puppy will catch the kitten after 36 seconds.