80
The question implies that you're counting by tens & the number comes after 70 and before 90. This is the only number that fits this description.
Using logical concept, the value of the number in question will be 80.
Let the number be n ;
Hence, we have ;
70, n, 90
n - 70 = 10 - - (1)
90 - n = 10 - - - (2)
Using either of the two expressions, we can obtain the value of n :
n - 70 = 10
n = 10 + 70
n = 80
Hence, the number is 80
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