The goal in a Mad Veteranarian puzzle is to convert animals from one type to another. The applet provides a set of animal-conversion machines, and a list of puzzles to be solved. You can choose a puzzle from the PUZZLE CHOICE pop-up menu.

In this example, we are asked to convert three cats to seven mice. We will not be allowed to have any cats or dogs left over upon completion. We have available for our use machines that perform these conversions:
 Machine Number Machine’s Operation Shorthand 1 one cat into two dogs and five mice 1C 2D 5M 2 one dog into three cats and three mice 1D 3C 5M 3 one mouse into a cat and a dog 1M 1C 1D
Initially the worksheet contains 3C, which tells us we have three cats. The only machine we can use is machine one, since it is the only machine that can accept lone cats. Since it is the only possible move, and can only be used in the forward direction, the only move button enabled is its forward button (labeled 2D 5M). So we click on that button.

The result (see figure below) is shown as 1:1  2C 2D 5M. The first 1 is our move count; the second 1 indicates we used machine 1; the rest of the line means we now have two cats, two dogs, and five mice.

We have many more options for our second move. We could use machine 1 to convert another cat, or we could use machine 2 or 3 to convert one of the dogs or mice that we just created. We could use machine 1 in reverse, but that would just cancel out our first move. We also enough animals to use machine 3 in reverse. In fact, let's try this last option. We click on machine 3’s backward button (labeled 1M) to make another mouse from a cat and a dog.

The result is 1C 1D 6M. Note that the machine is listed as -3 to indicate we used machine 3 in reverse.

We only need one more mouse to reach our goal. Machine 3 can do that for us, and we have the necessary animals to run it in reverse. So we use machine 3 again to make another mouse from our last cat and dog.

The result is 7M, seven mice. Since this is our goal, the applet tells us the puzzle is solved.

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