Question 20008
Set three of the coins aside.


Weigh the remaining six coins (three on each side of the scale).  This will tell you whether the counterfeit coin is one of the three that you set aside or with one of the sets of three that you are weighing.


If the coin is with the three that you set aside, weigh two of those coins to find the conterfeit coin.


If the coin is with one of the other sets of three (you should know which set from the first weighing), take two of those coins and weigh them.  That will tell you which one of the three is the counterfeit coin.


So, it is possible to do it in just two weighings.