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Three friends brought 3 pens together each 10 dollars. Next day they got 5 dollars cash back so they shared each 1 dollar and donated 2 dollars. Now the pen cost for each guy will be 9 dollars (\$10 -\$1).
But if you add all 9+9+9 = 27 dollars and donated amount is 2 dollars so total 29 dollars.
Where is the other \$1?