Question : Directions: In the following question a series is given, with one or more number(s) missing. Choose the correct alternative from the given ones that will complete the series.
7, 51, 8, 65, 9, ?
Option 1: 79
Option 2: 80
Option 3: 81
Option 4: 82
Correct Answer: 81
Solution :
Given:
7, 51, 8, 65, 9, ?
Add 1 to the numbers at odd positions to get the next term at the odd position –
7 + 1 = 8; 8 + 1 = 9
Square the previous term and add natural numbers starting from 2 in decreasing order to get the numbers at the even position –
51→7
2
+ 2 = 51
65→8
2
+ 1 = 65
Thus, the missing number will be 9
2
+ 0 = 81.
So, 81 is the missing number of the series. Hence, the third option is correct.
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