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The formula for counting 3s is expressed as
i = 1 ∑ k ⌊ 3 i 1 0 0 ⌋
we normally stop when the floor function returns zero.
⌊ 3 1 0 0 ⌋ + ⌊ 9 1 0 0 ⌋ + ⌊ 2 7 1 0 0 ⌋ + ⌊ 8 1 1 0 0 ⌋
Anything ahead of 81 will return 0. Hence the sum is 48.