109.85 should round to 109.8
109.850000000000000000001 should round to 109.9
As I recall, the analogy was to think of it as a teeter-totter. Anything less than 5 tips it to the left, anything greater than 5 tips it to the right and exactly 5 is perfectly balanced in the middle, so you have to pick a side for it. By convention, you go with whatever makes the previous digit even.