Add a 0 to the end of a natural number to get a new number, which is 10 times of the original number, and half of it is 10/2=5 times of the original number.

Add a 0 to the end of a natural number to get a new number. Use half of this new number and four times the original natural number to get a five-digit number, which is 5+4=9 times the original number.

So this five-digit number should be a multiple of 9.

A multiple of 9. The sum of the digits of each digit is a multiple of 9.

1+9+2+8=20。 Because the smallest number on a number is 0, the largest number is 9, and 20 plus a number is a multiple of 9, then this number can only be 7.

So this five-digit number is 17928.

The original natural number is 17928/9= 1992.