**Types of Number Series:** Some of the important types of series are given below.

**Addition Series:** In this type of series, a number is added to the previous element to get the next element. The number which is added can be the same for the whole series or in a certain order.

**Example(1):** What will come in the place of question mark (?) in the given series?

**2, 4, 6, 8, 10, ?**

**Answer:** In the given series the difference between two consecutive elements is same. In this series, the number 2 is added in the previous element to get the next one. $$ 2 \overset{+2}{\longrightarrow}\ 4 \overset{+2}{\longrightarrow}\ 6 \overset{+2}{\longrightarrow}\ 8 \\ \overset{+2}{\longrightarrow}\ 10 \overset{+2}{\longrightarrow}\ \boxed{12} $$

**Example(2):** What will come in the place of question mark (?) in the given series?

**1, 2, 4, 7, 11, ?**

**Answer:** In the given series the difference between two consecutive elements is in the increasing order. In this series, the numbers 1, 2, 3, 4, and 5 are added to the previous element respectively, to get the next element in the increasing order. $$ 1 \overset{+1}{\longrightarrow}\ 2 \overset{+2}{\longrightarrow}\ 4 \overset{+3}{\longrightarrow}\ 7 \\ \overset{+4}{\longrightarrow}\ 11 \overset{+5}{\longrightarrow}\ \boxed{16} $$

**Subtraction Series:** In this type of series, a number is subtracted from the previous element to get the next element. The number which is subtracted can be the same for the whole series or in the increasing order.

**Example(1):** What will come in the place of question mark (?) in the given series?

**16, 13, 10, 7, ?**

**Answer:** In the given series the difference between two consecutive elements is same. In this series, the number 3 is subtracted from the previous element to get the next one. $$ 16 \overset{-3}{\longrightarrow}\ 13 \overset{-3}{\longrightarrow}\ 10 \overset{-3}{\longrightarrow}\ 7 \\ \overset{-3}{\longrightarrow}\ \boxed{4} $$

**Example(2):** What will come in the place of question mark (?) in the given series?

**18, 17, 15, 12, ?**

**Answer:** In the given series the difference between two consecutive elements is in the increasing order. In this series, the numbers 1, 2, 3, and 4 are subtracted from the previous element respectively, to get the next element in the increasing order. $$ 18 \overset{-1}{\longrightarrow}\ 17 \overset{-2}{\longrightarrow}\ 15 \overset{-3}{\longrightarrow}\ 12 \\ \overset{-4}{\longrightarrow}\ \boxed{8} $$