Answer:
6 Chocolate bars and c marshmallows would be needed to completely use all the 12 graham crackers.
We can be able to make 6 s'mores.
Step-by-step explanation:
2G + 1C + 1M → S
According to reaction , 2 graham crackers along with 1 chocolate bar and 1 marshmallow gives 1 s'more.
Then for 12 graham crackers will combine with:

Then for 12 graham crackers will combine with:

Then 12 graham crackers will give:

We can be able to make 6 s'mores.