How do you solve this question?

Question

Player 1 runs to first base at a speed of 20 ft/s while Player 2 runs from second base to third base at a speed of 15 ft/s. Let s be the distance between the two players. How fast is s changing when Player 1 is 60 ft from home plate and Player 2 is 30 ft from second base?

ds/dt = ?

 Note: distance between bases is 90 ft. (see diagram).

Answer 1

s = 90 + 20t - 15t