Solution:

400

Explanation:
Begins with 15
2*15-1 => 29
2*29-2 => 56
2*56-4 => 108
2*108-8 => 208
2*208-16 => 400

Solution:

52

sum of consecutive prime numbers
(2+3 , 5+7 , 11+13 , 17+19 , 23+29)

Solution:

20(Twenty) : when you write them , these are only numbers having vowel 'E'

Solution:

The answer is C.

These are letter of the question asked in reverse order.

Solution:

6 (the number of letters in the month august)

January : 7 letters
February: 8 letters
March : 5 letters
April : 5 letters
May : 3 letters
June : 4 letters
July : 4 letters

Solution:

192 021

If you look at it as two-digit numbers you'll see:10 11 12 13 14 15 16 17 18 19 20 21

Solution:

Next in sequence is 13112221.

This series describes each number that came before it:
1 [One]
11 [One One]
21 [Two One(s)]
1211 [One Two and One One(s)]
111221 [One One, One Two, and Two One(s)]
312211 [Three One(s), Two Two(s), and One One]

The next number in the series is:
13112221 [One Three, One One, Two Two(s), Two One(s)]

Solution:

60.

100 years in a century, 365 days in a year, 24 hours in a day, 60 minutes in an hour, & 60 seconds in a minute.

Solution:

'2'

This single sequence in fact consists of two Series:
The first Series is 2-3-4-5-...
and
The second Series is 9-18-36-72-... .

Solution:

The next one is 8
77 : 7*7 = 49
49 : 4*9 = 36
36 : 3*6 = 18
18 : 1*8 = 8