[House's eyes narrow. Multiple answers, okay. He runs through a few different base systems in his head, then does a little more mental math before settling on another answer after about half a minute of silence.]
Roman numerals. X is ten, V is five, I is one. Twelve is X-I-I, seven is V-I-I, half of twelve.
private
Roman numerals. X is ten, V is five, I is one. Twelve is X-I-I, seven is V-I-I, half of twelve.