Question 3
There are 13 coins. Two of them are counterfeit and both have equal weights. The other 11 coins are genuine and have with equal weights as well, but they have a different weight from that of the counterfeit coins. We are not trying to identify the counterfeit coins but instead, we want to determine whether a counterfeit coin is heavier or lighter than a genuine coin. How can this be done in three weighings on a two-pan balance?