Digits

How many natural numbers greater than 4000 which are formed from the numbers 0,1,3,7,9 with the figures not repeated,

B) How many will the number of natural numbers less than 4000 and the numbers can be repeated?

Correct result:

a =  144
b =  375

Solution:

$a=2\cdot 4\cdot 3\cdot 2+4\cdot 4\cdot 3\cdot 2\cdot 1=144$

$b=5+4\cdot 5+4\cdot 5\cdot 5+2\cdot 5\cdot 5\cdot 5=375$

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