Numbers with exactly two set bits

How many integers $n$ with $1 \le n \le 100$ have exactly two set bits in their binary representation (i.e. popcount(n) == 2)?

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Two set bits $= 2^i + 2^j$ for distinct positions - count the pairs, don't scan.
Usable positions are $0..6$ (since $2^7=128>100$ ); every pair $\le 96$ , so the answer is $\binom{7}{2}$ .

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Asked at: Game-Based Aptitude, Mixed Quant & Coding