Answer:
PPF, PFF
Step-by-step explanation:
There are several ways you can list all the possible combinations. A couple of my favorite are a) use a binary counting sequence; b) use a gray code counting sequence.
Using the first method, the binary numbers 000 to 111 can be listed in numerical order as 000, <em>001</em>, 010, <em>011</em>, 100, 101, 110, 111. Letting 0=P and 1=F, the ones missing from your list are the ones in italics in my list.
Using the second method, we change the right-most character, then the middle one, and finally the left-most character so there is one change at a time: 000, <em>001</em>, <em>011</em>, 010, 110, 111, 101, 100.
After you have a list of all possible combinations, it is a simple matter to compare the given list to the list of possibilities to see which are missing.
