3 of them.
Three. The trailing two zeros are not significant.
Yes
890
900
300, 600 and 900.
Three digits can't possibly add up to 900. The largest possible sum of 3 digits is (9 + 9 + 9) = 27.
850
450 or 900
144, 324, 576, 900
900 of them.
10x10x9 = 900 ans.
There are 900 6-digit palindromes.