Hi ga821,Your starting numbers are right:
6 ways for a thousands digit from {4,5,6,7,8,9} and
9 ways for the odd digit (any digit except the repeated one). The trouble is the arrangement factor and a missing case.
The overcount: don't arrange identical digitsWhen three of the digits are the
same, you never rearrange them among themselves - swapping two identical digits gives you the
same number. The only thing that actually moves is the
single odd digit. So there's no
3! and no extra
×2 here; multiplying by
3*2 double-counts numbers that are literally identical.
Also, a quick arithmetic check:
6*9*3*2 = 324, so
324 -
1 =
323, not
215 - the formula as written doesn't land on the answer.
Why you actually need two separate casesThe ">4000" rule forces the thousands digit to be
4-
9, and that constraint behaves differently depending on
where the odd digit sits:
-
Odd digit is NOT in the thousands place (thousands is part of the triple):
6 for the repeated digit ×
9 for the odd digit ×
3 positions for the odd digit among the last three slots =
162.
-
Odd digit IS the thousands digit (last three are all the repeat):
6 for the thousands digit ×
9 for the repeated digit ×
1 (the other three slots are forced) =
54.
Total =
162 +
54 =
216, then subtract the single number
4000 -
215.
So the fix is: replace your
×3×2 with just "where does the one odd digit go," and add the case where that odd digit is the leading
4-
9.
Lock in the identical-digit ideaHow many distinct numbers use the digits
5,
5,
5,
7? It's
not4! = 24. Just list them:
- 5557, 5575, 5755, 7555 - only
4.
You only ever decide
where the odd digit goes - the identical 5's don't create new arrangements. That's exactly the overcount your extra factor introduced.
Answer: Bga821
How I did it, kindly correct if wrong-
first digit can be chosen in 6 ways (4, 5, 6, 7, 8, 9), the second and third digit in 1 way because it has to be a repeat of the first digit.
the last digit can be chosen in 9 ways (excluding the digit chosen in the first three digits)
now the other two repeating digits which are similar to the first one can be given positions in 3! ways
the only thing one has to keep in mind is it is greater than 4000 hence it becomes 6*9*3*2 - 1 = 215