Logarithmic Inequality Integer Solutions

CAT 2022 Slot 2 · QA · Hard · Algebra

The number of distinct integer values of nn satisfying 4log2n3log4n<0\frac{4 - \log_2 n}{3 - \log_4 n} < 0, is

Answer

47

Explanation

Rewrite log4n=log2n2\log_4 n = \frac{\log_2 n}{2}. Let x=log2nx = \log_2 n. The inequality becomes: 4x3x2<0    4x6x2<0    x4x6<0\frac{4 - x}{3 - \frac{x}{2}} < 0 \implies \frac{4 - x}{\frac{6 - x}{2}} < 0 \implies \frac{x - 4}{x - 6} < 0. This inequality holds when 4<x<64 < x < 6. Substituting x=log2nx = \log_2 n: 4<log2n<6    24<n<26    16<n<644 < \log_2 n < 6 \implies 2^4 < n < 2^6 \implies 16 < n < 64. Integer values of nn are 17,18,,6317, 18, \dots, 63. Number of integers = 6317+1=4763 - 17 + 1 = 47.

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