How many 7 digit number are there having the digit 3 three times & the digit 0 four times ?
a) 15
b) 3^3 * 4^4
c) 18
d) none of the above

• Answer is (a). 15
  The first digit on the left for 7 digit would always be fixed to 3 (if 0 the first digit then it will no longer makes a 7 digit number).
  So there are only 6 places left which is to be filled by number 3 two times and number 0 four time.
  So possible arrangements at 6 place for repetitive numbers would be = 6! / 4!*2! = 15
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