Number-series-example16

 

Find the missing number in the following set:

3, 10, 24, x, 108, 220 

Scroll down to see the answer

 

 

 

 

 

Correct answer  is 52

 

3, 10, 24, x, 108, 220 

We can solve it in 2 ways.

Α way

 

 Calculate the differences between 

those that are next to each other.

10-3=7

24-10=14

108-52=56

220-108=112

 

The number series of the differences is:

7,14, x, 56, 112

Notice that every term is multiple by 2

7X2=14        56X2=114

The number series is GP with common ratio 2

So the number series of differences is:

7, 14, 28, 56,  112

 

Therefore the missing number is:

24+28=52

 

B way

 The rule is as follows:

 add 2 to the previous number then multiply times 2.

(3+2)x2=10

(10+2)x2=24

(24+2)x2=52

(52+2)x2=108

(108+2)x2=220

Correct answer is:

52

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Number series test 2-Alternating series

 

One type of number series that often appears in competitive exams is the alternating series.

 

  What is Alternating number series

 This type of series may consists of two series combined into a single series. The alternating terms of this series may form an independent series in itself

 The alternating series are quite difficult. You need to make many examples to get acquainted and get into their spirit and be able to deal with them in the exams. Many exercises-practice will give you the confidence to succeed in the competitive exams.

Examples

 

 Question1: Look carefully at the following series.What pair comes next?

 46, 43, 40, 38, 35, 32, 30,...?,,,,,?

This is an alternating subtraction series in which 3 is subtracted twice, then 2 is subtracted once, then 3 is subtracted twice, and so on. So the next terms in the following series will be 27, 24

 Question2: Look carefully at the following series.What pair comes next?

 18, 8, 18, 10, 18, 12, 18, ...

 

 In this alternating repetition series, the random number 18 is interpolated every other number into an otherwise simple addition series that increases by 2, beginning with the number 8.

12+2=14 

18 is interpolated every other number

Answer is 14, 18 

 

  Question3: Look carefully at the following series.What pair comes next?

 9,18,10,20,12,24,... 

 This is an alternating multiplication and subtracting series: First, multiply by 2 and then subtract 8. 

9X2=18   18-8=10   10X2=20  20-8=12     12X2=24

So  24-8=16     16x2=32

 Answer is 16, 32

 

Practice with selective questions 

 Alternating number series test

Detailed solutions

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By Panos Kalliantasis