Answer:
20 seconds
Step-by-step explanation:
The speed of A is:
sA = 18 units / 3 seconds = 6 units/second
The speed of B is:
sB = -15 units / 3 seconds = -5 units/second
The relative speed is:
s = sA − sB
s = 6 units/second − (-5 units/second)
s = 11 units/second
The time for the distance between them to be 220 units is:
220 units = 11 units/second × t
t = 20 seconds