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We conducted a poll asking for "What would you like to read?". The results were in favour of Mental Ability and Quantitative Aptitude. Thank you for your votes, and here we are to prove that your opinion counts. This is our first post on Mental Ability covered under the Topic Reasoning.

## (FOR BANK PO, SSC, B. Ed. ASPIRANTS)

Most of the number series can be solved by addition and subtraction (Multiplication and Division being the Supersets for these). In plain English that means you need to find what to add or subtract to find the next (or missing) number in the series.

We will understand the concept by solving a few questions and then you can attempt some on your own.

Example 1: Find the next number in the series: 1, 3, 6, 10, 15, __

Looking at the series we find that the series is an "INCREASING" sequence.

So we try to find what is being added. Here the series is

1, 1+2, 1+2+3, 1+2+3+4

or Add 1, 2, 3, 4 and so on to the last number.

1+2 =3, 3+3 = 6, 6+4 = 10, 10+5 = 15

So the answer is 15+ 6 = 21.

Practice Question 1: 21, 25, 33, 49, 81, __

(a) 145 (b) 129 (c) 113 (d) 97

Ans (a)

Example 2: Find the next number in the series: 87, 80, 74, 69, 65, __

Looking at the series we find that the series is a "DECREASING" sequence.

So we try to find what is being subtracted. The series is

87, 87 - 7 = 80, 80 - 6 = 74, 74 - 5 = 69, 69 - 4 = 65

So the answer is 65 - 3 = 62

Practice Question 2: 66, 36, 21, __

(a) 13.5 (b) 14.5 (c) 13 (d) 14

Ans (a)

Example 3: Find the next number in the series: 40, 15, 30, 25, 20, 35, __

Looking at the series we find that the series is a "MIXED" sequence.

By mixed series we mean that it is composed of two different series.

i.e. 40, 30, 20, ... (decreasing) and 15, 25, 35, .. (increasing)

The trick here is to know which series will give us the answer. We see that the missing number belongs to the decreasing series. Ignoring the other series the answer is

20 - 10 = 10

Practice Question 3: 7.5, 5, 15, 4, 30, 3, 60, 2, __

(a) 1 (b) 75 (c) 120 (d) 135

Ans (c)

Some Sample Questions (Easy to Difficult, No Options Given for Now, you don't need them)

1. 8, 20, 32, 44, __

2. 55, 44, 33, 22, __

3. 63, 57, 52, 48, __

4. 100, 85, 75, 70, __

5. 96, 102, 114, 132, __

6. 17, 26, 44, 80, __

7. 10, 13, 12, 15, 14, __, __ (two missing numbers)

8. 12, 7, 18, 10, 24, 13, __

9. 2, 5, 11, 23, 47, 95, __

10. 1, 4, 9, 16, 25, 36, __

We will continue with this post in coming days. Please leave a comment if you have a question or you want us to post something specific.