The least needed is:7.
First, 1,2,3,4=x
Next, 5,6,7,8=y
Also, 9,10,11.12=z
3 weighings...
Then, weigh 1,2,5,9=a; 3,4,6,10=b; 5,6,11,12=c.
Another 3 weighings.
If you do guess and check, you will find some number pairs can be two numbers...
Ex-1,3 or 5,6
The 7th weighing is 1,4,5,7=d
With that measurement, you can confirm your guess and check.
QuyNguyen2013
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