1 + 1/3 + 1/5 ... + 1/(2N-1).

Each time you tie two together the number of string ends available decreases by 2. The chance of grabbing a string and its end is 1/(2N-1).

Grass.

16.

38, Because mary is 2 years older than her sister if she is 4 and her sister is 2

13
because 7+7=14 and 14-1=13

5.
He makes 4 cigarettes with the 16 butts. Then he smokes them and has 4 more butts to make another cigarette.

Glasses.

Two oranges.

She did not count how many she had correctly. She really had 25,009

They all have the same amount of weight. ONE POUND of Feathers. ONE POUND of meat. And ONE POUND of rocks!

Mosquitoes. They add to misery, subtract from pleasure, multiply quickly, and divide your attention.

Any number.