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How many natural number are possible by using digits 0,1,3,7,9 (repetition is allowed) which are less than 7000.?

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Here, all the five digit number will be greater than 7000, so we choose only 4 digits out of 5 to make the number.

So,

Natural numbers less than 7,000

\underline{1^{st}}, \underline{2^{nd}}, \underline{3^{rd}}, \underline{4^{th}}

There are 3 ways to fill position 1 (i.e. 0,1,3) because a 4 digit number starting from 7 and 9 would be greater than 7000. 

and for the remaining position, there are 5 ways each so,

3\times 5\times 5\times 5 -1 = 375

As 0000 is not included,

Number of Natural numbers less than 7,000 = 375 - 1 = 374.

Posted by

Pankaj Sanodiya

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