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Combi Thoughts

Combi Thoughts

Here is the question-

1257 is a number where 1<2<5<7. Now the question is how many four digit numbers are there that satisfies this property?

The answer-

It's really a simple one. Don't think for the order first. Let me explain. See, if you take any four digits you will find that they are always in a order. So it makes sense that you only have to find four digits from the nine digits & that is 9C4.

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