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Bijection Principle from I.S.I. Entrance

Bijection principle is an important tool in combinatorics. This problem from I.S.I Entrance is useful for Math Olympiad. Try video, sequential hints and practice problems.

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This problem of Bijection Principle is from B.Stat, B.Math Entrance.

How many natural numbers less that $10^8$ are there, whose sum of digits equals 7?