Practice Exercise 1: Find the wrong term in the following number series.

8.4 , 8.2 , 8.6 , 7.8 , 9.4 , 6.4 , 12.6

A. 8.2

B. 7.8

C. 6.4

D. 12.6

E. 9.4

8.4 – 0.2 = 8.2

8.2 + 0.4 = 8.6

8.6 – 0.8 = 7.8

7.8 + 1.6 = 9.4

9.4 – 3.2 = 6.2 ≠ 6.4

6.2 + 6.4 = 12.6

Therefore, 6.4 is the wrong term. So, C is the correct option.

Practice Exercise 2: Find the wrong term in the following number series.

2 , 3 , 8 , 27 , 112 , 570 , 3396

A. 3

B. 570

C. 3396

D. 8

E. 112

2 × 1 + 1 = 3

3 × 2 + 2 = 8

8 × 3 + 3 = 27

27 × 4 + 4 = 112

112 × 5 + 5 = 565 ≠ 570

565 × 6 + 6 = 3396

Therefore, 570 is the wrong term. So, B is the correct option.

Calculation Tips:

You should calculate 27 × 4 as (80 + 28) or (120 – 12).

> 565 × 6 = 500 × 6 + 60 × 6 + 5 × 6 = 3000 + 360 + 30 = 3390 

Practice Exercise 3: Find the wrong term in the following number series.

602 , 598 , 591 , 563 , 498 , 372 , 155

A. 155

B. 591

C. 498

D. 598

E. 372

Observe the difference between the consecutive terms

602 – 598 = 4

598 – 591 = 7

591 – 563 = 28

563 – 498 = 65

498 – 372 = 126

372 – 155 = 217

Note that we can write 217 as 63 + 1

Similarly, 126 = 53 + 1

65 = 43 + 1

28 = 33 + 1

Following this pattern, the next difference should be  23 + 1 = 9 ≠ 7. Therefore, the wrong term is 598. It should be 591 + 9 = 600.

Now, 602 – 600 = 2 = 13 + 1. So, D is the correct option.

Practice Exercise 4: Find the wrong term in the following number series.

7 , 4 , 5 , 9 , 21 , 52.5 , 160.5

A. 52.5

B. 21

C. 5

D. 9

E. 4

7 × 0.5 + 0.5 = 4

4 × 1 + 1 = 5

5 × 1.5 + 1.5 = 9

9 × 2 + 2 = 20 ≠ 21

20 × 2.5 + 2.5 = 52.5

52.5 × 3 + 3 = 160.5

Therefore, 21 is the wrong term. So, B is the correct option.

Practice Exercise 5: Find the wrong term in the following number series.

130 , 60 , 34 , 18 , 10 , 6 , 4

A. 60

B. 18

C. 6

D. 10

E. 4

Taking from the opposite side

4 × 2 – 2 = 6

6 × 2 – 2 = 10

10 × 2 – 2 = 18

18 × 2 – 2 = 34

34 × 2 – 2 = 66 ≠ 60

66 × 2 – 2 = 130

Therefore, 60 is the wrong term. So, A is the correct option.

Practice Exercise 6: What should come in place of question mark (?) in the following letter series?

AC , FH , KM ,  PR , (?)

A. TV

B. XV

C. WU

D. VX

E. UW

> To get the first alphabet of a term, add 3 to the position of the second alphabet of the previous term.

> To get the second alphabet of a term, add 2 to the position of the first alphabet of that term.

A (B) (D E) F (G) H (I J) K (L) M (N O) P (Q) R (S T) U (V) W

Therefore, the next term is UW. So, E is the correct option.

Practice Exercise 7: What should come in place of question mark (?) in the following letter series?

BZ , HT , NN , (?) , ZB

A. PT

B. TH

C. TQ

D. PH

E. None of these

> To get the first alphabet of a term, add 6 to the position of the first alphabet of the previous term.

> To get the second alphabet of a term, subtract 6 from the position of the second alphabet of the previous term.

Following this pattern, the next term is TH. So, B is the correct option.