You have 12 coins, one of which is fake. the fake coin is indistinguishable from the rest except that it is heavier. can you determine which is the real coin heavier using a balance scale and only 3 weighings?

1st weighing)
Get all 12 coins. put 6 in one pan and 6 in the other. 
They will NOT balance.  Get the 6 coins from the heavier pan.

2nd weighing
Separate these into 3 coins in one pan 3 in the other.
These will NOT balance.  Get the 3 coins from the heavier pan.

3rd weighing
Separate these 3 coins by placing one coin in each pan.
Do they balance? If they do, the heavy coin is the one NOT being weighed.
If they don't balance, the heavy coin is in the heavy pan.

21

step-by-step explanation:

21-4

The surface area would be 576 because you multiply all the numbers then multiply that number by 6