A five digit number can be filled by ten digits (0, 1, 2, 3, 4, 5, 6, 7, 8, 9) in 10 ways.
Thus, each of the five places can be filled in 10 ways.
10 x 10 x 10 x 10 x 10 = 100000 ways
Now, we need to exclude the possibility of 0 existing at ten-thousand place
The number of 5-digits numbers that have 0 at ten-thousand place = 10 x 10 x 10 x 10 = 10000
Therefore, the numbers of ways to write 5-digits numbers = 100000 – 10000 = 90000