Tiffiny ate 1 slice of cake. omer ate 2 slices. if tiffiny ate 1/5 of the cake and all the slices were the same size, what fraction of the cake remained after tiffiny and omer had eaten?

If the total of the slices is one cake, and each slice is 's' in size, then you could model an equation like this:
Total (one cake) = 1 slice + 2 slices + 1/5 slices + remaining cake, or

Now since I'm sure this question is incomplete, plug in whatever values you need to get the answer.