Question

12=18,23=27,31=9,89=? Which number replace the question mark?

Answer 1

Given : 12=18, 23=27, 31=9, 89=?

To Find :  Which number replace the question mark

Solution:

12 = 18

12 = 21  - (1  + 2)  = 21 - 3  = 18

23 = 27

23 = 32 - ( 2 + 3)  = 32 - 5 = 27

31 = 9

31 = 13 - ( 3  + 1)  =  13 - 4  = 9

89 = ?

89 =  98  - ( 8 + 9)  =  81

ab =  ba - (a + b)   = 10b + a - a - b  =  9b

so another logic is 9 multiplied by unit digit

ab = b * 9

12   = 2 * 9 = 18

23 = 3 * 9 = 27

31  = 1 * 9  = 9

89 = 9 * 9  = 81

81 replaces the question mark

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