For this case what we should do is write the equation of movement for each person.
We have then:
For Roger:
r = 20 * t + 500
For Willa:
r = 40 * t
Equating the equations:
20 * t + 500 = 40 * t
We cleared t:
40 * t - 20 * t = 500
20 * t = 500
t = 500/20
t = 25 s
Answer:
it did take willa 25 s to catch up Roger.