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This brute force approach in Python runs in 141ms (under PyPy).

Solution:The seven-figure number is 2939979.Considered as an addition sum, we get a slightly longer, but much faster program. This version runs in 37ms.

Can the seven-digit number be 3996999 ?

No. 3996999 ÷ 3 = 1332333, which is a 7 figure number (not a 6 figure number). The result of operations on 3996999 is 999699 (a 6 figure number), and the two are clearly not equal.