The difference series is divided into three module .

A constant difference

An increasing difference

A decreasing difference

A constant difference

In constant difference, the difference between every two consecutive term is constant i.e.,same.

Example: 1,5,9,13,17___
A) 20
B) 21
C) 22
D) 23

Answer:C) 21

Explanation:
Here the difference between every two consecutive term is constant i .e., 4 so the answer is 17+4=21

Example: 100,95,90,85___
A) 75
B) 70
C) 80
D) 105

Answer:C) 80

Explanation:
Here the difference between every two consecutive term is constant i .e. 5 and in decreasing order so answer is 85-5=80

Increasing difference

In "increasing difference", the difference between consecutive terms keeps on either increasing order.

Example: 5,6,8,11,15___
A) 20
B) 22
C) 23
D) 25

Answer:A) 20

Explanation:
Here the difference between first and second term is 1, difference between 2nd and third term is 2 and so on. That is,the difference is increasing by a constant 1.So the final answer is 15+5=20

Example: 2,5,8,11,14___
A) 20
B) 17
C) 23
D) 25

Answer:B) 17

Explanation:
Here the difference between first and second term is 3, difference between 2nd and third term is 3 and so on. That is,the difference is increasing by a constant 3.So the final answer is 14+3=17

Decreasing difference

In "decreasing difference", the difference between consecutive terms keeps on either decreasing order.

Example: 55,45,36,28___
A) 21
B) 22
C) 23
D) 25

Answer:A) 21

Explanation:
Here the difference between first and second term is 10, difference between 2nd and third term is 9 and so on. That is,the difference is decreasing by a constant 1. So the final answer is
28-7=21

Example: 963,927,855,747,603,423___
A) 209
B) 208
C) 207
D) 206

Answer:C) 207

Explanation:
Here the difference between first and second term is (36 x 1), difference between 2nd and third term is (36 x 2) and so on. That is,the difference is increasing by a constant (36 x 6).So the final answer is 207