Question 843623
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Hi,
Find the number of four-digit counting numbers containing at least one zero:
<u>All</u> minus  <u>four-digit counting numbers with no zero</u>
Note: zero cannot be the first digit
(a) Repeated digits are allowed. {{{9*10*10*10- 9*9*9*9}}} 
(b) repeated digits are not allowed. {{{9*9*8*7 - 9*8*7*6 }}}