More numerates

Spent yesterday with Bratii Familus and my parents, Bratii Minor was messing about with a four-legged stool flipping it and trying to turn it into a seat.

"Be careful with that, do you know how old it is?", asked my father.
"No", he replied.
"Well have a guess"
"10 years?"
"Nope"
"7?"
"Nope, wrong way"
"100 years"
"That's a bit of a jump", says Bratus Pater. "What's halfway between the two?"
"55?"
"Nope", says my father.
"45?"
"Yes", agrees my father.
But he wasn't paying attention so I injected "Did you hear that? It's 45 years old"
"No it isn't" says my father.
"But you just said it was."
"No I was agreeing that 45 was halfway between 10 and 100."
"No it isn't/It's 55" comes simultaneously from both me and Bratus Pater.
"Of course it is - 100 minus 10 is 90 and half of 90 is 45."
"And add 10" adds Bratus Pater.
"Why?"
"Because it's halfway."
At this point both Bratus Major and my mother agree that halfway between 10 and 100 is 45 and an argument ensues.
"Woah, woah", I call. "What's halfway between 5 and 10?"
"7 and a half", replies Major instantly.
"And what's 10 minus 5?"
"5."
"And half of 5 is?
"2 and a half"
"So using the previous argument 2 and a half is halfway between 5 and 10"
"Ah... no"
"So what do you have to do"
"Add 5"
"Tada"
"So I was right then" pipes up Bratus Minor.
"Yes you were" I tell him.

Scary, very scary.

Oh and as it turns out the stool is approximately 44 years old [sigh]