93. There are 10 coins, 9 of which are genuine and of the same weight, and one is counterfeit and lighter than the others. What is the minimum number of weighings on a balance scale without weights that are needed to find the counterfeit coin?
$\triangle$ Let's number the coins. Place coins numbered $1,2,3$ on the left pan and coins numbered $4,5,6$ on the right pan. We can use Figure 30 for further steps.

If the scales balance, the counterfeit coin is among the remaining four. It takes two more weighings to determine the counterfeit coin from these four (see problem 91).

If the scales are unbalanced, the counterfeit coin is in the lighter of these two sets of three. One more weighing is needed to identify the counterfeit coin.

In general, three weighings are required. But could we have managed with two? Let's try placing four coins on each pan (other options are clearly worse). However, the counterfeit coin could be in one of these sets of four, which would still require two more weighings, totaling three.

Answer: three.