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Aboyhasnoname
Bunuel is there a better approach for this?

I had to make the subsets...

d has to be atleast 6...
So 6 can be achieved in ....1 + 2 + 3 ...Can arrange this in 3! ways..
if d is 7..Then 1 + 2 + 4 ...3! ways again ....
d is 8...then 1 + 2 + 5 ..or 1 + 3 + 4 ....each 3! ways..
d is 9 ...then 1 + 2 + 6 or 1 + 5 + 3 or 2 + 3 + 4 ..all in 3! ways..Answer comes to 42 ...but I took a lot of time and I had no confidence..as i thought there could be some combination that I am not able to get...Is there a way to find that out?..What different digits when added together gives a particular different digit? or we have to do that manually, like the way I did...?



Aboyhasnoname Looking at your work, I want to first say - your approach is absolutely correct and you got the right answer! The case-by-case method you used is actually THE standard approach for this type of problem. Let me help you build confidence in your method and show you how to verify completeness.

Your Method is Perfect - Here's Why:

You correctly identified that \(d\) must be at least \(6\) (since minimum sum = \(1+2+3=6\)) and at most \(9\) (single digit limit). This systematic case analysis is exactly right!

Systematic Framework to Ensure Completeness:

Step 1: Determine the range of \(d\)
- Minimum: \(1+2+3=6\) ✓
- Maximum: \(9\) (single digit) ✓
- Therefore: \(d \in \{6, 7, 8, 9\}\)

Step 2: For each value of \(d\), find partitions
Here's the systematic way to ensure you don't miss combinations:

For \(d=6\): Need three distinct digits summing to \(6\)
- Start with smallest: \(\{1,2,?\}\) → \(?=3\) → \(\{1,2,3\}\) ✓
- Try next: \(\{1,3,?\}\) → \(?=2\) (already used) ✗
- Count: \(1\) combination × \(3!=6\) arrangements = \(6\)

For \(d=7\): Need three distinct digits summing to \(7\)
- \(\{1,2,?\}\) → \(?=4\) → \(\{1,2,4\}\) ✓
- \(\{1,3,?\}\) → \(?=3\) (duplicate) ✗
- Count: \(1\) combination × \(3!=6\) arrangements = \(6\)

For \(d=8\): Need three distinct digits summing to \(8\)
- \(\{1,2,?\}\) → \(?=5\) → \(\{1,2,5\}\) ✓
- \(\{1,3,?\}\) → \(?=4\) → \(\{1,3,4\}\) ✓
- \(\{2,3,?\}\) → \(?=3\) (duplicate) ✗
- Count: \(2\) combinations × \(3!=6\) arrangements = \(12\)

For \(d=9\): Need three distinct digits summing to \(9\)
- \(\{1,2,?\}\) → \(?=6\) → \(\{1,2,6\}\) ✓
- \(\{1,3,?\}\) → \(?=5\) → \(\{1,3,5\}\) ✓
- \(\{1,4,?\}\) → \(?=4\) (duplicate) ✗
- \(\{2,3,?\}\) → \(?=4\) → \(\{2,3,4\}\) ✓
- Count: \(3\) combinations × \(3!=6\) arrangements = \(18\)

Total: \(6+6+12+18=42\)

Quick Verification Technique:
To be absolutely sure you haven't missed combinations, use this checklist:
  1. For each \(d\), start with smallest possible first digit (\(1\))
  2. Systematically increase the second digit
  3. Calculate the required third digit
  4. Check if all three are distinct and valid (\(1-9\))

Why There's No "Magic Formula":
For partition problems like this, case-by-case IS the systematic approach. The key is being methodical (like you were!) rather than looking for a shortcut that doesn't exist.

Time-Saving Tip:
Once you find the combinations for each \(d\), you know each gives \(3!=6\) arrangements. So you can quickly write:
- \(d=6\): \(1\) combo
- \(d=7\): \(1\) combo
- \(d=8\): \(2\) combos
- \(d=9\): \(3\) combos
- Total: \((1+1+2+3) \times 6 = 42\)

Your solution was perfect - trust your systematic approach! The only thing you needed was confidence that you covered all cases, which you absolutely did.

You can practice similar combinatorics problems here. Look for counting and partition questions to build confidence with this systematic case-by-case method.
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Let abcd be a general four-digit number and all the digits are non-zero.

How many four-digits numbers abcd exist such that the four digits are all distinct and such that a + b + c = d ?

Case 1: (a,b,c,d) = {1,2,3,6} : Number of such numbers = 3! = 6
Case 2: (a,b,c,d) = {1,2,4,7} : Number of such numbers = 3! = 6
Case 3: (a,b,c,d) = {1,2,5,8} : Number of such numbers = 3! = 6
Case 4: (a,b,c,d) = {1,2,6,9} : Number of such numbers = 3! = 6
Case 5: (a,b,c,d) = {1,3,4,8} : Number of such numbers = 3! = 6
Case 6: (a,b,c,d) = {1,3,5,9} : Number of such numbers = 3! = 6
Case 7: (a,b,c,d) = {2,3,4,9} : Number of such numbers = 3! = 6

Total four-digits numbers abcd exist such that the four digits are all distinct and such that (a + b + c = d) = 6*7 = 42

IMO E
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