Number series questions

What will come next in the below number series:

A)    4,  6,  12,  18,  30,  42,  60,  72, ?
B)    22, 21, 23, 22, 24, 23, ?, ?
C)    5, 6, 7, 8, 10, 11, 14, ?, ?

Solution:

Let me confess that Questions related to number series are not asked very frequently in companies. Only few companies have such questions in their written test (for professionals with 0-2 years of experience).

Having said that, lets have a look at the questions above:

A) Answer: 102

This series represents average of twin prime numbers (Prime numbers whose difference is just 2).

4  = (3+5)/2
6  = (5+7)/2
12 = (11+13)/2
18 = (17+19)/2
30 = (29+31)/2 ...

So the next three number in the series will be (101 + 103)/2 = 102

B) Answer: 25, 24

The series is

-1, +2, -1, +2 ...

or you may consider it as two AP series

22, 23, 24, ...

and

21, 22, 23, ...

C) Answer: 15, 19

This series is also mix of two interleaved sequences:

5,  7,  10,  14,  19
    +2 +3 +4 +5

and

6,  8,  11,  15
    +2 +3 +4