How many 6-digit numbers can be formed using the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, if repetitions of digits are allowed?

There are 6 digits.  Each digit can take ten different numbers except for the first digit since it cannot be zero.

So:

9 x 10 x 10 x 10 x 10 x 10

900000 numbers.

Another way of thinking about this is to just count up to 999,999.  Obviously there are 999,999 different numbers here.  But since our number has to have 6 digits in them, we have to delete 99,999 numbers.  Thus there are 900,000 different numbers.