A stalagmite that is currently 254.7 mm tall has been observed to grow by 0.2 mm per year. Assuming the growth has been constant, how many years ago was the stalagmite 245 mm tall?

1. Calculate the difference in height between the current height and the past height:
254.7 - 245 = 9.7 \, \text{mm}
2. Divide the difference in height by the growth rate to find the number of years ago:
9.7 \, \text{mm} / 0.2 \, \text{mm/year} = 48.5 \, \text{years}
3. The stalagmite was 245 mm tall 48.5 years ago.

The answer is 48.5 years.