Do you mean 12 km/h because this is a measurement of distance
Let t be the time they both meet. When they meet they both have covered the distance between them.
Arthur with 3 m/s would have covered 3t m
Betty with 2 m/s would have covered 2t m
3t + 2t = 100
5t = 100
t = 100/5
t = 20 seconds.
Spot who is helplessly runing back and forth at 5 m/s would have done this for 20 seconds.
This distance covered by spot = 5m/s * 20s = 100m