Mike spent 37 dollars on notebooks.  He bought a total of 12 notebooks.  He bought 3 red notebooks at 4 dollars each, 2 green notebooks at 2 dollars each, and the rest were blue notebooks.  How much does each blue notebook cost?
He bought 12-3-2=<<12-3-2=7>>7 blue notebooks.
He paid 3*$4=$<<3*4=12>>12 for red notebooks
He paid 2*$2=$<<2*2=4>>4 for green notebooks
He spent $37-$12-$4=$<<37-12-4=21>>21 on blue notebooks.
Each blue notebook costs $21/7=$<<21/7=3>>3.
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