1. A car is white. Sampling from the domains of cars I have seen on the road: 3%.
2. A car is a white, ten year old Ford with a dent on the rear right door Ditto: 10^-9.
3. A ten-mile car trip will involve a collision. 10^-7 or thereabouts.
4. A building is residential. Off the cuff, close to even odds.
5. A person is below the age of 20. 5%.
6. A word in a book contaains a typo. Any given word in a published book: 10^-8. Any given book: 10%.
7. Your arm will spontaneously transform into a blue tentacle today. Negligible, dominated by fundamental errors in understanding a la you-are-in-the-Matrix scenarios - 10^-20 is almost certainly too high, 1/10^^100 might be too low.
8. A purse contains exactly 71 coins. 0.1%.
9. 76297 is a prime number. 10%.

Unfortunately, memories of degrees of confidence tend to come back badly distorted, unless they're crystallized somehow. Worse, they tend to come back consistently biased towards whatever would be judged correct now, which makes them useless or worse. Numbers crystallize those memories, making them usable and enabling you to retrace steps

Really? Mythbusters fans might disagree. :P