99. There are 6 identical-looking coins, but 4 are genuine, of the same weight, while 2 are counterfeit, lighter, and also weigh the same. What is the minimum number of weighings on a balance scale without weights that are needed to find both counterfeit coins?
$\triangle$ As usual, let's number the coins. During the first weighing, we will place coins numbered $1,2,3$ on the left pan, and all the others on the right (Fig. 33).

The scales may balance. Then each of the triplets contains one counterfeit coin. To find them, we need two more weighings (see the solution to problem 90).

The scales may be unbalanced. In this case, both counterfeit coins are contained in one triplet - the lighter one (in this case - on the left pan). One more weighing will be required.

Answer: in three.

![](https://cdn.mathpix.com/cropped/2024_05_21_0aa0e6ac168e80891953g-044.jpg?height=595&width=1342&top_left_y=245&top_left_x=357)