Can you find the odd number in the following?

2716, 4135, 5321 and 7145

What number comes next in this sequence:
7 8 5 5 3 4 4 _

This puzzle is for Harry Potter fans. Do you know why Tom Riddle is called Lord Voldemort?

Twice ten are six of us,

Six are but three of us,

Nine are but four of us;

What can we possibly be?

Would you know more of us?

Twelve are but six of us,

Five are but four, do you see?

What are we?

I have an eye but still cannot see.

Who am I?

Two in a corner, one in a room, zero in a house, but one in a shelter. What am I?

I am a mother. However, I never give birth.

I am a father. However, I never nurse my children.

Wandering is not possible for me, but standing still happens occurs rarely.

Who am I?

A plane crashed between the border of Mexico and America. Where do you bury the survivors?

Replace the question mark with the correct number, given the pair of numbers exhibits a similar relationship?

? : 3839 :: 11 : 1209

John has played 50 ODI's and his average is 50. How many runs should he score in his 51st ODI, so that his average score jumps to 51?