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Q: How many four-digits numbers can be formed under the following conditions?
(a) Leading digits cannot be zero.
(b) Leading digits cannot be zero and no repetition of digits is allowed.
(c) Leading digits cannot be zero and the number must be a multiple of 5.

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(a) Since the leading digit cannot be zero, the number of ways the leading place can be filled by any digit is 9 (from 1-9). The remaining three digits can hold any of the 10 digits.
Therefore total number of 4 digit numbers where leading digit cannot be zero
= 9*10*10*10 = 9000

(b) Since the leading digit ...