It walks on 4 when its young, walks on 2 when it's older, and walks on 3 when its very old?

Find The Next Number in the series:

12 13 15 17 111 113 117 119 123 ?

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

? : 3839 :: 11 : 1209

I know a nine-letter word having only one vowel.
Do you ?

Can you find out the total number of triangles in the given figure?

What always comes into a house through the keyhole?

Christina is practising her dance steps along with her friends. In a particular sequence, all of them form a row. At that point, Niharika is standing in the 4th position from either end of the row.
Can you find out how many girls are practising together?

Before the start of the car race, John and Jacob have the same amount of fuel in their car. With this fuel, John can drive for 4 hours while Jacob can drive five hours.
After a time they realize that the amount of fuel left in John's car is 1/4th of the fuel in Jacob's.

For how long they are racing?

The below mentioned letters have been placed in a order using a logic.

DEAD, BALL, FEAR, SHOP

Can you find the logic?

I travel all around the world but always stay in the corner. What am I?