I'm not 100% sure that I'm right here but I tried
100,000 years = 2 km = 200,000 cm
50,000 years = 1 km = 100,000 cm
100,000 years / 200,000 cm = 0.5 cm per year
I feel like I may have done something wrong here but 0.5 cm multiplied by 100,000 gives you 50,000 years and multiplied by 200,000 gives you 100,000 years.
